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Filed Pursuant to Rule 424(b)(5)

Registration No. 333-296607

P R O S P E C T U S S U P P L E M E N T
(To Prospectus dated June 8, 2026)

[MISSING IMAGE: lg_fuelcellenergyvert-4c.jpg]

10,714,286 Shares

FuelCell Energy, Inc.

Common Stock

We are offering 10,714,286 shares of our common stock.

Our common stock is listed on The Nasdaq Global Market under the symbol “FCEL.” On July 6, 2026, the last reported sale price for our common stock was $29.73 per share.

Investing in our common stock involves a high degree of risk. You should read this prospectus supplement and the accompanying prospectus carefully before you make your investment decision. See “Risk Factors” beginning on page S-4 of this prospectus supplement, as well as the documents we file with the Securities and Exchange Commission that are incorporated by reference in this prospectus supplement and in the accompanying prospectus, including our most recent Annual Report on Form 10-K and any subsequently filed Quarterly Reports on Form 10-Q and Current Reports on Form 8-K, for more information.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

	Per Share			Total
Public offering price		$	21.00		$	225,000,006
Underwriting discounts and commissions(1)		$	1.05		$	11,250,000
Proceeds, before expenses, to us		$	19.95		$	213,750,006

(1)

See the section entitled “Underwriting” for a description of the compensation payable to the underwriters.

We have granted the underwriters an option for a period of 30 days from the date of this prospectus supplement to purchase an additional 1,607,143 shares of our common stock. If the underwriters exercise the option in full, the total underwriting discounts and commissions payable by us will be $12,937,500 and the total proceeds to us, before expenses, will be $245,812,509.

The underwriters expect to deliver the shares of common stock against payment on or about July 9, 2026.

Joint Lead Book-Running Managers

Citigroup

Barclays

Book-Running Managers

Oppenheimer & Co.

RBC Capital Markets

Goldman Sachs & Co. LLC

Co-Managers

Canaccord Genuity

B. Riley Securities

BMO Capital Markets

Siebert Williams Shank

Tuohy Brothers

July 7, 2026.

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Prospectus Supplement

	ABOUT THIS PROSPECTUS SUPPLEMENT AND PROSPECTUS					S-ii
	CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS					S-iii
	PROSPECTUS SUPPLEMENT SUMMARY					S-1
	THE OFFERING					S-3
	RISK FACTORS					S-4
	USE OF PROCEEDS					S-6
	CAPITALIZATION					S-7
	DILUTION					S-9
	DESCRIPTION OF CAPITAL STOCK					S-11
	MATERIAL UNITED STATES FEDERAL INCOME TAX CONSIDERATIONS					S-18
	UNDERWRITING					S-23
	LEGAL MATTERS					S-29
	EXPERTS					S-29
	WHERE YOU CAN FIND ADDITIONAL INFORMATION					S-29

Prospectus

	ABOUT THIS PROSPECTUS					1
	FORWARD-LOOKING STATEMENTS					1
	INCORPORATION BY REFERENCE					2
	OUR COMPANY					3
	RISK FACTORS					4
	USE OF PROCEEDS					4
	THE SECURITIES WE MAY OFFER					4
	DESCRIPTION OF CAPITAL STOCK					5
	DESCRIPTION OF DEBT SECURITIES					12
	DESCRIPTION OF WARRANTS					20
	DESCRIPTION OF UNITS					21
	SELLING SECURITY HOLDERS					22
	PLAN OF DISTRIBUTION					23
	LEGAL MATTERS					25
	EXPERTS					25
	WHERE YOU CAN FIND ADDITIONAL INFORMATION					25

We are offering to sell, and are seeking offers to buy, the securities only in jurisdictions where such offers and sales are permitted. The distribution of this prospectus supplement and the accompanying prospectus and the offering of the securities in certain jurisdictions may be restricted by law. Persons outside the United States who come into possession of this prospectus supplement and the accompanying prospectus must inform themselves about and observe any restrictions relating to the offering of the securities and the distribution of this prospectus supplement and the accompanying prospectus outside the United States. This prospectus supplement and the accompanying prospectus do not constitute, and may not be used in connection with, an offer to sell, or a solicitation of an offer to buy, any securities offered by this prospectus supplement and the accompanying prospectus to or by any person in any jurisdiction in which it is unlawful for such person to receive or make such an offer or solicitation.

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ABOUT THIS PROSPECTUS SUPPLEMENT AND PROSPECTUS

This document consists of two parts. The first part is this prospectus supplement, which describes the specific terms of the securities we are offering. The second part is the accompanying prospectus, which gives more general information, some of which may not apply to this offering. This prospectus supplement and the information incorporated by reference in this prospectus supplement may add to, update and change information contained in, or incorporated by reference into, the accompanying prospectus. To the extent there is a conflict between the information contained in this prospectus supplement, on the one hand, or in any document incorporated by reference herein that was filed with the Securities and Exchange Commission (“SEC”) before the date of this prospectus supplement, on the other hand, you should rely on the information in this prospectus supplement. If any statement in one of these documents is inconsistent with a statement in another document having a later date, for example, a document incorporated by reference in this prospectus supplement or the accompanying prospectus, the statement in the document having the later date modifies or supersedes the earlier statement.

The accompanying prospectus is part of a registration statement that we filed with the SEC using a shelf registration process. Under the shelf registration process, from time to time, we may offer and sell any of the securities described in the accompanying prospectus separately or together with other securities described therein.

You should read this prospectus supplement along with the accompanying prospectus, the documents incorporated by reference in this prospectus supplement and the accompanying prospectus and any related free writing prospectus that we authorized to be distributed to you carefully before you invest. These documents contain important information you should consider when making your investment decision. You should also read and consider the information in the documents to which we have referred you under the heading “Where You Can Find Additional Information” on page S-29 of this prospectus supplement and on page 25 of the accompanying prospectus and under the heading “Incorporation by Reference” on page 2 of the accompanying prospectus.

Neither we nor the underwriters have authorized anyone to provide you with any information other than the information contained in or incorporated by reference into this prospectus supplement and the accompanying prospectus and any related free writing prospectus that we authorized to be distributed to you or to which we have referred you. If anyone provides you with different or additional information, you should not rely on it. Neither we nor the underwriters take any responsibility for, or can provide any assurance as to the reliability of, any other information that others may give you.

Neither we, nor the underwriters, nor anyone acting on our behalf is making an offer to sell these shares of common stock in any jurisdiction where the offer or sale is not permitted, and you should not consider this prospectus supplement or the accompanying prospectus to be an offer or solicitation relating to the securities in any jurisdiction in which such an offer or solicitation relating to the securities is not authorized. You should assume that the information contained in this prospectus supplement, the accompanying prospectus, any related free writing prospectus that we have authorized to be delivered to you and the documents incorporated by reference herein and therein is accurate only as of their respective dates, regardless of the time of delivery of such documents or of any sale of securities. Our business, financial condition, results of operations and prospects may have changed since those dates. Furthermore, you should not consider this prospectus supplement or the accompanying prospectus to be an offer or solicitation relating to the securities if the person making the offer or solicitation is not qualified to do so, or if it is unlawful for you to receive such an offer or solicitation.

We further note that the representations, warranties and covenants made by us in any agreement that is filed as an exhibit to any document that is incorporated by reference into this prospectus supplement or the accompanying prospectus were made solely for the benefit of the parties to such agreement, including, in some cases, for the purpose of allocating risk among the parties to such agreements, and should not be deemed to be a representation, warranty or covenant to you. Moreover, such representations, warranties or covenants were accurate only as of the date when made. Accordingly, such representations, warranties and covenants should not be relied on as accurately representing the current state of our affairs.

For purposes of this prospectus supplement and the accompanying prospectus, references to “FuelCell Energy,” “we,” “our,” “us,” “our Company” and “the Company” are to FuelCell Energy, Inc. and our consolidated subsidiaries, unless otherwise indicated or the context otherwise requires.

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CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS

This prospectus supplement and the accompanying prospectus, the documents incorporated by reference herein and therein and any related free writing prospectuses issued by us may contain forward-looking statements, within the meaning of the Private Securities Litigation Reform Act of 1995 (the “PSLRA”), about our financial condition, results of operations, plans, objectives, expectations, future performance and business. Statements preceded by, followed by or that include words such as “expects,” “anticipates,” “estimates,” “goals,” “projects,” “intends,” “plans,” “believes,” “predicts,” “should,” “seeks,” “will,” “could,” “would,” “may,” “forecast,” or words or phrases of similar import are intended to identify some of the forward-looking statements and are included, along with this statement, for purposes of complying with the safe harbor provisions of the PSLRA. Forward-looking statements are neither historical facts, nor assurances of future performance. Instead, such statements are based only on our beliefs, expectations and assumptions regarding the future. As such, the realization of matters expressed in forward-looking statements involves inherent risks and uncertainties. The following factors, among others, could cause actual results and future events to differ materially from those set forth or contemplated in the forward-looking statements:

•

general risks associated with product development and manufacturing;

•

general economic conditions;

•

changes in interest rates, which may impact project financing;

•

supply chain disruptions;

•

changes in the utility regulatory environment;

•

changes in the utility industry and the markets for distributed generation, distributed hydrogen, and fuel cell power plants configured for carbon capture or carbon separation;

•

potential volatility of commodity prices that may adversely affect our projects;

•

availability of government subsidies and economic incentives for alternative energy technologies;

•

our ability to remain in compliance with U.S. federal and state and foreign government laws and regulations;

•

our ability to maintain compliance with the listing rules of The Nasdaq Stock Market;

•

rapid technological change;

•

competition;

•

the risk that our bid awards will not convert to contracts or that our contracts will not convert to revenue;

•

market acceptance of our products;

•

changes in accounting policies or practices adopted voluntarily or as required by accounting principles generally accepted in the United States;

•

factors affecting our liquidity position and financial condition;

•

government appropriations;

•

the ability of the government and third parties to terminate their development contracts at any time;

•

the ability of the government to exercise “march-in” rights with respect to certain of our patents;

•

our ability to successfully market and sell our products internationally;

•

our ability to develop additional commercially viable products in the future;

•

our ability to implement our strategy;

•

our ability to reduce our levelized cost of energy and deliver on our cost reduction strategy generally;

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•

our ability to protect our intellectual property;

•

litigation and other proceedings;

•

the risk that commercialization of our new products will not occur when anticipated or, if it does, that we will not have adequate capacity to satisfy demand;

•

our need for and the availability of additional financing;

•

our ability to generate positive cash flow from operations;

•

our ability to service our long-term debt;

•

our ability to increase the output and longevity of our platforms and to meet the performance requirements of our contracts; and

•

our ability to expand our customer base and maintain relationships with our largest customers and strategic business allies.

Any forward-looking statements are subject to risks and uncertainties, known and unknown, that could cause actual results and future events to differ materially from those set forth in or contemplated by the forward-looking statements, including, without limitation, the risks and uncertainties described in or incorporated by reference into this prospectus supplement, including under the caption “Risk Factors,” and the accompanying prospectus. Any forward-looking statement contained in this prospectus supplement, the accompanying prospectus, the documents incorporated by reference herein and therein, and any related free writing prospectuses issued by us speaks only as of the date on which the statement was made, and we undertake no obligation to update publicly or revise any forward-looking statements, whether oral or written, for any reason, whether as a result of new information, future events or otherwise. In addition to the factors outlined above, new risks emerge from time to time, and we cannot predict all of the risks that may impact our business or the extent to which any particular risk, or combination of risks, may cause actual results to differ materially from those contained in any forward-looking statement.

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PROSPECTUS SUPPLEMENT SUMMARY

The following summary highlights basic information about FuelCell Energy and this offering. Because it is a summary, it does not contain all of the information that may be important to you. You should review this entire prospectus supplement and the accompanying prospectus carefully, including our consolidated financial statements and other information incorporated by reference in this prospectus supplement and the accompanying prospectus, before making an investment decision. In addition, please carefully read the “Risk Factors” section beginning on page S-4 of this prospectus supplement, the accompanying prospectus, our Annual Report on Form 10-K for the fiscal year ended October 31, 2025 and any amendment or update thereto reflected in subsequent filings with the SEC and incorporated by reference in this prospectus supplement and the accompanying prospectus.

Our Business

FuelCell Energy is a clean energy technology company and a stationary fuel cell manufacturer with 22 years of operating experience in this field. We were founded in 1969 to research and develop electrochemical power generation technology and are headquartered in Danbury, Connecticut. Since the early 2000s, we have been manufacturing, selling and servicing our proprietary molten carbonate fuel cell systems, which deliver large-scale, continuous clean power and advanced emissions management. Unlike traditional power generation methods that rely on combustion, our fuel cells generate electricity electrochemically through a chemical reaction rather than burning fuel, resulting in ultra-low emissions and high efficiency. Our carbonate fuel cell systems are fuel-flexible, with the ability to run on biofuels, renewable natural gas, or hydrogen-hydrocarbon blends, and provide reliable baseload power, carbon capture, and thermal energy for chilling, heating, and process steam. As global energy demand rises driven by artificial intelligence, electrification, and the need for enhanced grid resiliency, we believe solutions like ours will be vital in addressing next-generation needs, helping to strengthen the grid, reducing pollution, and supporting decarbonization goals. We have proven utility-scale projects operating at 10 megawatts (“MW”), 20 MW, and 58.8 MW, with an average of over 10 years of continuous run time. As a company, we are motivated by our purpose of enabling a world empowered by clean energy.

We target a range of markets and applications with our products, including utilities and independent power producers, data centers, wastewater treatment, commercial and hospitality, and microgrids, among others. We market our products primarily in the U.S. and Canada, the European Union and the United Kingdom, and priority Asian markets including South Korea, Singapore, Malaysia, and Thailand. We selectively pursue additional opportunities in other regions that align with our strategic priorities. We focus our expansion on markets and regions that value clean distributed generation, have poor grid reliability and/or challenged transmission and distribution lines, and can benefit from the value streams our products provide.

In addition to our existing core molten carbonate-based commercial products, we engage strategically in research and development, both company-funded and carried out under grants from and commercial agreements with private companies and various government agencies through our Advanced Technologies programs. Our Advanced Technologies programs are currently focused on continued development and advancement of our core carbonate fuel cell technology as well as commercialization of our solid oxide electrolysis technology for distributed hydrogen. We focus on generating revenue from our core recurring and non-recurring revenue sources, while working to identify the next trends in clean energy we believe we can commercialize, take to market, and grow into future revenue streams.

Recent Events

Fit Energy USA LP Transaction and Warrants

On June 22, 2026, we entered into a Capital Equipment Purchase Agreement (the “CEPA”) with Fit Energy USA LP (“Fit”), by its general partner, Fit US Inc. Pursuant to the CEPA, the Company agreed to manufacture, sell, and deliver to Fit carbonate fuel cell block systems (each, a “Block”), with each Block having a nameplate generating capacity of 2.5 MW, for a total aggregate generating capacity of up to 380 MW across four phases. The fuel cell systems are intended to supply baseload electricity for data center applications. The payment obligations with respect to the initial phase, representing a generating capacity of

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30 MW in phase 0, became effective upon execution of the CEPA. Thereafter, Fit will have the ability to elect, at its sole option, to proceed with the remaining phases for generating capacity of 100 MW in phase 1, generating capacity of 125 MW in phase 2 and generating capacity of an additional 125 MW in phase 3, in each case, with a milestone-based payment obligation with an initial deposit due at election of each phase, upon delivery by Fit of timely election notices.

On June 22, 2026, in connection with the CEPA, we issued warrants to purchase up to 12,000,000 shares of our common stock to Fit, comprised of three equal tranches of warrants, each to purchase up to 4,000,000 shares of our common stock. The exercise price of these warrants is $26.44 per share, and the warrants are subject to performance-based vesting tied to non-refundable deposits by Fit pursuant to the CEPA in connection with phases 1, 2 and 3. Any warrant that has not vested within 24 months of the date of issuance will automatically terminate and be cancelled. Once a tranche of warrants has vested, that tranche of warrants will expire if not exercised within 24 months following the applicable vesting date for such tranche.

The holders of the warrants are entitled to certain resale shelf registration rights pursuant to a registration rights agreement, dated June 22, 2026, between us and Fit, with respect to the shares of common stock issuable upon exercise of the warrants held by Fit from time to time. Pursuant to the registration rights agreement, the Company is required to file a registration statement under the Securities Act of 1933, as amended (the “Securities Act”), providing for the resale of all or a part of the common stock issuable upon exercise of the warrants held by Fit within 30 calendar days following the closing of the transactions under the CEPA.

Export-Import Bank of the United States Financing

On June 29, 2026, we announced that the Board of Directors of the Export-Import Bank of the United States approved a financing package on June 23, 2026 of $49 million to be disbursed in two tranches.

The first tranche, disbursed on June 30, 2026, provided the Company with net proceeds of approximately $22 million after financing fees and customary expenses and reserves to support the delivery of five 2.8 MW Blocks to Gyeonggi Green Energy Co., Ltd. in South Korea. A second tranche is expected to be disbursed in October 2026, subject to customary closing conditions.

Corporate Information

Our principal executive offices are located at 3 Great Pasture Road, Danbury, Connecticut 06810. Our telephone number is (203) 825-6000. We maintain a website at www.fuelcellenergy.com. The information on, or that can be accessed through, our website is not incorporated by reference in this prospectus supplement or in the accompanying prospectus, and you should not consider it to be a part of this prospectus supplement or the accompanying prospectus. Our website address is included as an inactive textual reference only.

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THE OFFERING

Issuer

FuelCell Energy, Inc.

Common stock offered by us

10,714,286 shares of common stock (or 12,321,429 shares of common stock if the underwriters’ option to purchase additional shares is exercised in full).

Common stock outstanding after this offering

78,322,459 shares (or 79,929,602 shares if the underwriters’ option to purchase additional shares is exercised in full)(1).

Use of proceeds

We estimate that the net proceeds from this offering will be approximately $213.4 million (or approximately $245.4 million if the underwriters’ option to purchase additional shares is exercised in full), after deducting underwriting discounts and commissions and our estimated offering expenses.

We currently intend to use the net proceeds from this offering for capital expenditures related to expansion of manufacturing capacity to support growth, working capital and general corporate purposes.

See “Use of Proceeds” on page S-6 for additional information.

Nasdaq Global Market symbol

FCEL

Risk factors

An investment in our common stock involves a high degree of risk. See “Risk Factors” beginning on page S-4 of this prospectus supplement and other information included in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference in this prospectus supplement and the accompanying prospectus for a discussion of factors you should carefully consider before deciding to invest in our common stock.

(1)

The number of shares of common stock outstanding after this offering is based on 67,608,173 shares of common stock outstanding as of June 30, 2026, and does not include, in each case as of June 30, 2026:

•

12,000,000 shares of common stock reserved for issuance upon exercise of our outstanding warrants, with an exercise price of $26.44 per share;

•

1,261 shares of common stock reserved for issuance upon conversion of all of the issued and outstanding shares of our 5% Series B Cumulative Convertible Perpetual Preferred Stock (“Series B Preferred Stock”);

•

1,469,571 shares of common stock reserved for issuance upon time-based vesting of restricted stock units granted under our equity incentive plan;

•

636,288 shares of common stock reserved for potential issuance in connection with the settlement of performance-based restricted stock units granted under our equity incentive plan, with the issuance of such shares (and the number of shares to actually be issued, if any) being contingent on achievement of certain stock price performance goals;

•

439 shares of common stock reserved for future issuance upon exercise of outstanding options to purchase common stock, with a weighted average exercise price of $574.67;

•

2,318,983 shares of common stock reserved for future grants of awards under our equity incentive plan; and

•

300,078 shares of common stock reserved for future issuance under our employee stock purchase plan.

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RISK FACTORS

An investment in our common stock involves a high degree of risk. Prior to making a decision about investing in our securities, you should carefully consider the specific risk factors discussed below as well as the risk factors discussed under the sections entitled “Risk Factor Summary” and “Risk Factors” in our Annual Report on Form 10-K for the fiscal year ended October 31, 2025 and any subsequently filed Annual Reports on Form 10-K, Quarterly Reports on Form 10-Q, Current Reports on Form 8-K and other filings we make under the Securities Exchange Act of 1934, as amended (the “Exchange Act”), each of which is incorporated by reference in this prospectus supplement in its entirety, together with all of the other information contained or incorporated by reference in this prospectus supplement and the accompanying prospectus, the documents incorporated by reference herein and therein, and any related free writing prospectuses we may issue. The risks and uncertainties we have described are not the only ones we face. Additional risks and uncertainties not presently known to us or that we currently deem immaterial may also affect our operations. If any such risks actually occur, our business, financial condition, or results of operations could be materially and adversely affected. In such cases, the trading price of our common stock could decline, and you may lose all or part of your investment.

Risks Related to this Offering

Management will have broad discretion as to the use of the proceeds from this offering, and we may use the proceeds in ways in which you and other stockholders may disagree.

We have not designated any amount of the net proceeds we will receive from this offering for any particular purpose. Accordingly, our management will have broad discretion as to the application of such net proceeds and could use them for purposes other than those contemplated at the time of this offering. Our stockholders may not agree with the manner in which our management chooses to allocate and spend such net proceeds. In addition, such proceeds could be applied in ways that do not improve our operating results or increase the value of your investment. Our failure to apply these funds effectively could have a material adverse effect on our business, delay the development and commercialization of our products and cause the price of our common stock to decline.

Investors in this offering will suffer immediate and substantial dilution.

Because the price per share of common stock in this offering will be substantially higher than the pro forma net tangible book value per share of our common stock, investors in this offering will suffer immediate and substantial dilution. See the section entitled “Dilution” below for a detailed illustration of the dilution you will incur if you participate in this offering.

You may experience future dilution as a result of future equity offerings or the exercise of our outstanding options or warrants.

In order to raise additional capital, we may in the future offer additional shares of our common stock or other securities convertible into or exchangeable for our common stock at prices that may not be the same as the price per share in this offering. We may sell shares of our common stock or other securities convertible into or exchangeable for our common stock in any other offering at a price per share that is less than the price per share paid by investors in this offering, and investors purchasing shares of our common stock or other securities in the future could have rights superior to existing stockholders. The exercise of our outstanding options and warrants may adversely affect our stock price due to sales of a large number of shares of our common stock or the perception that such sales could occur. These factors also could make it more difficult to raise funds through future offerings of our securities and could adversely impact the terms under which we could obtain additional equity capital. Exercise of our outstanding options and warrants or any future issuance of additional shares of our common stock or other equity securities, including but not limited to options, warrants, restricted stock units or other derivative securities convertible into our common stock, may result in significant dilution to our stockholders and may decrease our stock price.

The Company’s executive officers and directors have entered into lock-up agreements with the underwriters under which such individuals have agreed, and the Company has agreed, subject to certain exceptions, not to offer, sell, contract to sell, pledge or otherwise dispose of, directly or indirectly, any shares of our common stock or any securities convertible into or exercisable or exchangeable for our common stock without the permission of Citigroup Global Markets Inc. and Barclays Capital Inc. (on behalf of the

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underwriters) for a period of 60 days following the date of this prospectus supplement. We refer to such period as the lock-up period. When the lock-up period expires, we and our executive officers and directors will be able to sell our common stock in the public market, subject to compliance with applicable securities laws restrictions. In addition, Citigroup Global Markets Inc. and Barclays Capital Inc. (on behalf of the underwriters) may, in their sole discretion, release all or some portion of the shares of common stock subject to these lock-up agreements at any time and for any reason. Sales of a substantial number of such shares of common stock upon expiration of the lock-up or otherwise, the perception that such sales may occur, or early release of these agreements, could cause the market price of our common stock to fall.

Fluctuations in the price of our common stock, including as a result of actual or anticipated sales of shares by us and/or our directors, officers or stockholders, may make our common stock more difficult to resell.

The market price and trading volume of our common stock have been, and may continue to be, subject to significant fluctuations due not only to general stock market conditions, but also to changes in sentiment in the market regarding the industry in which we operate, our operations, business prospects or liquidity, or this offering. In addition to the risk factors discussed in our periodic reports and in this prospectus supplement, the price and volume volatility of our common stock may be affected by actual or anticipated sales of common stock by us and/or our directors, officers or stockholders, whether in the market, in connection with business transactions, in this offering or in subsequent public offerings. Stock markets in general have at times experienced extreme volatility unrelated to the operating performance of particular companies. These broad market fluctuations may adversely affect the trading price of our common stock, regardless of our operating results.

As a result, these fluctuations in the market price and trading volume of our common stock may make it difficult to predict the market price of our common stock in the future, cause the value of your investment to decline and make it more difficult to resell our common stock.

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USE OF PROCEEDS

We estimate that the net proceeds to us from this offering will be approximately $213.4 million (or approximately $245.4 million if the underwriters’ option to purchase additional shares from us is exercised in full), after deducting underwriting discounts and commissions and estimated offering expenses payable by us.

Due to the fact that we regularly review and evaluate our liquidity needs for various purposes, we do not believe we can provide the approximate amounts of the net proceeds that will be allocated to each or any of these purposes with certainty. As such, we have not specifically allocated the net proceeds among these purposes as of the date of this prospectus supplement. Such decisions will depend on market conditions and our liquidity needs as they evolve over time. Pending their use, we intend to invest the net proceeds to us from this offering in short-term, investment grade, interest bearing instruments or hold them as cash.

Our management will have broad discretion in the allocation of the net proceeds of this offering, and investors will be relying on the judgment of our management with regard to the use of these net proceeds.

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CAPITALIZATION

The following table sets forth our cash and cash equivalents and capitalization as of April 30, 2026:

•

on an actual basis;

•

on a pro forma basis, after giving effect to the issuance and sale subsequent to April 30, 2026 of 4,057,098 shares of our common stock for net proceeds of approximately $52,920,000, under our Open Market Sale AgreementSM with Jefferies LLC, B. Riley Securities, Inc., Barclays Capital Inc., BMO Capital Markets Corp., BofA Securities, Inc., Canaccord Genuity LLC, Citigroup Global Markets Inc. and Loop Capital Markets LLC (“Open Market Sale Agreement”); and

•

on a pro forma as adjusted basis to reflect the issuance and sale of shares of our common stock offered by us in this offering at a public offering price of $21.00 per share, assuming the full exercise by the underwriters of their option to purchase an additional 1,607,143 shares of common stock.

You should read this table together with “Use of Proceeds” in this prospectus supplement, and “Management’s Discussion and Analysis of Financial Condition and Results of Operations” and our consolidated financial data, each included in our most recent Annual Report on Form 10-K and most recent Quarterly Report on Form 10-Q incorporated by reference in this prospectus supplement. Cash and cash equivalents are not components of our total capitalization.

	As of April 30, 2026
(in thousands)	Actual			Pro Forma (giving effect to sales under the Open Market Sale Agreement)			Pro Forma As Adjusted for Offering
	(unaudited)
Cash and cash equivalents, unrestricted		$	373,167		$	426,087		$	671,500
Long-term debt and financing obligations, including current portion of long-term debt, long-term deferred revenue, operating lease liabilities and debt and other liabilities		$	169,062		$	169,062		$	169,062
Stockholders’ equity:
Common stock ($0.0001 par value); 1,000,000,000 shares authorized; 63,549,362 shares issued and outstanding, actual, 67,606,460 shares issued and outstanding, pro forma (giving effect to sales under the Open Market Sale Agreement) and 79,927,889 shares issued and outstanding, pro forma as adjusted			6			7			8
Additional paid-in capital			2,651,450			2,704,369			2,949,780
Accumulated deficit			(1,930,216)			(1,930,216)			(1,930,216)
Accumulated other comprehensive loss			(1,810)			(1,810)			(1,810)
Treasury stock, Common, at cost (57,681 shares)			(1,502)			(1,502)			(1,502)
Deferred compensation			1,502			1,502			1,502
Total stockholders’ equity:			719,430			772,350			1,017,762
Total capitalization		$	888,492		$	941,412		$	1,186,824

The number of shares of our common stock outstanding is based on 63,549,362 shares of our common stock outstanding as of April 30, 2026, and excludes as of April 30, 2026:

•

1,261 shares of common stock reserved for issuance upon conversion of all of the issued and outstanding shares of our Series B Preferred Stock;

•

1,457,508 shares of common stock reserved for issuance upon time-based vesting of restricted stock units granted under our equity incentive plan;

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•

652,855 shares of common stock reserved for potential issuance in connection with the settlement of performance-based restricted stock units granted under our equity incentive plan, with the issuance of such shares (and the number of shares to actually be issued, if any) being contingent on achievement of certain stock price performance goals;

•

439 shares of common stock reserved for future issuance upon exercise of outstanding options to purchase common stock, with a weighted average exercise price of $574.67;

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2,316,685 shares of our common stock reserved for future grants of awards under our equity incentive plan; and

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300,078 shares of common stock reserved for future issuance under our employee stock purchase plan.

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DILUTION

If you purchase any of the shares of common stock offered by this prospectus supplement, you will experience dilution to the extent of the difference between the offering price per share of common stock you pay in this offering and the adjusted pro forma net tangible book value per share of our common stock immediately after this offering.

Our historical net tangible book value as of April 30, 2026 was approximately $785.1 million, or approximately $12.35 per outstanding share of common stock. “Net tangible book value” is total assets, minus goodwill and intangible assets, minus total liabilities. “Net tangible book value per share” is net tangible book value divided by the total number of shares outstanding, assuming conversion of all outstanding shares of our Series B Preferred Stock into common stock.

Our pro forma net tangible book value as of April 30, 2026 was approximately $838.0 million, or approximately $12.40 per share. Our pro forma net tangible book value per share represents our historical net tangible book value as of April 30, 2026, after giving effect to our issuance and sale subsequent to April 30, 2026 of 4,057,098 shares of our common stock under our Open Market Sale Agreement for aggregate net proceeds to us of approximately $52.9 million.

After giving effect to the pro forma adjustments described above and the sale of 10,714,286 shares of our common stock in this offering at the public offering price of $21.00 per share, and after deducting the underwriting discounts and commissions and our estimated offering expenses, our adjusted pro forma net tangible book value as of April 30, 2026 would have been approximately $1,051.4 million, or approximately $13.42 per share of common stock. This represents an immediate increase in the pro forma net tangible book value of approximately $1.03 per share to our existing stockholders and an immediate dilution (i.e., the difference between the public offering price and the adjusted pro forma net tangible book value after this offering) to new investors purchasing shares in this offering of approximately $7.58 per share. The following table illustrates this calculation on a per share basis:

Public offering price per share			$	21.00
Pro forma net tangible book value per share as of April 30, 2026	$	12.40
Increase in pro forma net tangible book value per share attributable to the offering	$	1.02
Adjusted pro forma net tangible book value per share as of April 30, 2026, after giving effect to the offering			$	13.42
Dilution per share to new investors in the offering			$	7.58

If the underwriters were to fully exercise their option to purchase additional shares of common stock from us, the adjusted pro forma net tangible book value per share as of April 30, 2026, after giving effect to the offering, would be approximately $13.56 per share. This represents an increase in pro forma net tangible book value of approximately $1.16 per share to our existing stockholders and results in dilution of approximately $7.44 per share to investors purchasing shares in this offering at the public offering price.

The foregoing table is based on 67,607,721 shares of common stock outstanding (including 4,057,098 shares issued and sold subsequent to April 30, 2026 under our Open Market Sale Agreement). This calculation is on an “as converted” basis assuming the conversion of all outstanding shares of our Series B Preferred Stock into 1,261 shares of common stock. This number of shares excludes:

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12,000,000 shares of our common stock reserved for issuance upon exercise of our outstanding warrants, with an exercise price of $26.44 per share, issued subsequent to April 30, 2026;

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1,457,508 shares of our common stock reserved for issuance upon time-based vesting of restricted stock units granted under our equity incentive plan as of April 30, 2026;

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652,855 shares of our common stock reserved for potential issuance in connection with the settlement of performance-based restricted stock units granted under our equity incentive plan as of April 30, 2026, with the issuance of such shares (and the number of shares to actually be issued, if any) being contingent on achievement of certain stock price performance goals;

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439 shares of our common stock reserved for future issuance upon exercise of outstanding options to purchase common stock, with a weighted average exercise price of $574.67, as of April 30, 2026;

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2,316,685 shares of our common stock reserved for future grants of awards under our equity incentive plan as of April 30, 2026; and

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300,078 shares of our common stock reserved for future issuance under our employee stock purchase plan as of April 30, 2026.

To the extent that outstanding options or warrants are exercised, you will experience further dilution. In addition, we may choose to raise additional capital due to market conditions or strategic considerations, even if we believe that we have sufficient funds for our current or future operating plans. To the extent that additional capital is raised through the sale of equity or convertible debt securities, the issuance of these securities could result in further dilution to our stockholders.

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DESCRIPTION OF CAPITAL STOCK

General

The following description of our capital stock is a summary and does not purport to be complete. It is subject to, and qualified in its entirety by reference to, our Certificate of Incorporation, as amended (the “Certificate of Incorporation”), including the Amended Certificate of Designation for the 5% Series B Cumulative Convertible Perpetual Preferred Stock (the “Series B Certificate of Designation”), and our Third Amended and Restated By-laws (the “By-laws”), each of which is incorporated by reference as an exhibit to the registration statement of which this prospectus is a part. We encourage you to read our Certificate of Incorporation (including the Series B Certificate of Designation), our By-laws and the applicable provisions of the Delaware General Corporation Law (“DGCL”) for additional information.

Authorized and Outstanding Capital Stock

Under our Certificate of Incorporation, we are authorized to issue 1,000,000,000 shares of common stock, par value $0.0001 per share, and 250,000 shares of preferred stock, par value $0.01 per share, in one or more series designated by our board of directors, of which 105,875 shares of our preferred stock have been designated as 5% Series B Cumulative Convertible Perpetual Preferred Stock (“Series B Preferred Stock”).

As of June 30, 2026, 67,608,173 shares of our common stock were issued and outstanding and 64,020 shares of our Series B Preferred Stock were issued and outstanding. No other shares of our preferred stock were issued and outstanding.

As of June 30, 2026, we were obligated, if and when the holders exercise their conversion rights, to issue approximately 1,261 shares of our common stock upon conversion of all of the issued and outstanding Series B Preferred Stock.

As of June 30, 2026, there were outstanding options to purchase 439 shares of our common stock under our equity incentive plan; 1,469,571 shares of our common stock were reserved for issuance upon time-based vesting of outstanding restricted stock units granted under our equity incentive plan; 636,288 shares of our common stock were reserved for potential issuance in connection with the settlement of performance-based restricted stock units granted under our equity incentive plan; 2,318,983 shares of our common stock were available for future grants of awards under our equity incentive plan; and 300,078 shares of our common stock were reserved for future issuance under our employee stock purchase plan.

As of June 30, 2026, there were outstanding warrants to purchase up to 12,000,000 shares of our common stock held by Fit, comprised of three equal tranches of warrants, each to purchase up to 4,000,000 shares of our common stock. The exercise price of these warrants is $26.44 per share, and the warrants are subject to performance-based vesting tied to non-refundable deposits by Fit pursuant to the CEPA. Any warrant that has not vested within 24 months of the date of issuance will automatically terminate and be cancelled. Once a tranche of warrants has vested, that tranche of warrants will expire if not exercised within 24 months following the applicable vesting date for such tranche.

As of June 30, 2026, there were 20 holders of record of our common stock.

Common Stock

Voting Rights. The holders of our common stock have one vote per share. Holders of our common stock are not entitled to vote cumulatively for the election of directors. Generally, matters to be voted on by stockholders (including the election of directors in uncontested elections) must be approved by a majority of the votes properly cast on the matter at a meeting at which a quorum is present, subject to any voting rights granted to holders of any then-outstanding preferred stock. (The voting rights of the outstanding Series B Preferred Stock are described below under the heading “Series B Preferred Stock”). A plurality voting standard applies in contested director elections (i.e., when the number of nominees for election as directors exceeds the number of directors to be elected at such meeting).

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Stock”). Dividends consisting of shares of our common stock may be paid to holders of shares of our common stock. We have never paid a cash dividend on our common stock and do not anticipate paying any cash dividends on our common stock in the foreseeable future.

Liquidation Rights. In the event of our liquidation, dissolution or winding up, after payment of liabilities and liquidation preferences on any of our preferred stock then outstanding, the holders of shares of our common stock are entitled to share ratably in all assets available for distribution. (The liquidation and other rights of the outstanding Series B Preferred Stock are described below under the heading “Series B Preferred Stock”).

Other Rights. Holders of shares of our common stock (solely in their capacity as holders of shares of our common stock) have no preemptive rights or rights to convert their shares of our common stock into any other securities. There are no redemption or sinking fund provisions applicable to our common stock.

Listing on The Nasdaq Global Market. Our common stock is listed on The Nasdaq Global Market under the symbol “FCEL.”

Transfer Agent and Registrar. The transfer agent and registrar for our common stock is Equiniti Trust Company, LLC.

Series B Preferred Stock

Ranking. Shares of our Series B Preferred Stock rank with respect to dividend rights and rights upon our liquidation, winding up or dissolution:

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senior to shares of our common stock;

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junior to our debt obligations; and

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effectively junior to our subsidiaries’ (i) existing and future liabilities and (ii) capital stock held by others.

Dividends. The Series B Preferred Stock pays cumulative annual dividends of $50.00 per share, which are payable quarterly in arrears on February 15, May 15, August 15 and November 15. Dividends accumulate and are cumulative from the date of original issuance. Unpaid accumulated dividends do not bear interest.

The dividend rate is subject to upward adjustment as set forth in the Series B Certificate of Designation if we fail to pay, or to set apart funds to pay, any quarterly dividend on the Series B Preferred Stock.

No dividends or other distributions may be paid or set apart for payment on our common stock (other than a dividend payable solely in shares of a like or junior ranking), nor may any stock junior to or on parity with the Series B Preferred Stock be redeemed, purchased or otherwise acquired for any consideration (or any money paid to or made available for a sinking fund for such stock) by us or on our behalf (except by conversion into or exchange for shares of a like or junior ranking), unless all accumulated and unpaid dividends on the Series B Preferred Stock have been paid or funds or shares of common stock have been set aside for payment of such accumulated and unpaid dividends.

The dividends on the Series B Preferred Stock will be paid in cash, unless a registered holder elects (pursuant to the procedures set forth in the Series B Certificate of Designation) to receive such dividends in shares of our common stock. Any such shares of common stock paid in lieu of cash dividends will be treated as restricted securities and will not be transferable by the recipient thereof except pursuant to an effective registration statement or pursuant to an exemption from the registration requirements of the Securities Act.

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our assets. (For the avoidance of doubt, neither the voluntary sale of all or substantially all of our assets, nor a merger involving the Company, shall be deemed to be a voluntary or involuntary liquidation, dissolution or winding up of the Company.) As of April 30, 2026, the issued and outstanding shares of Series B Preferred Stock had an aggregate Liquidation Preference of $64.0 million.

Conversion Rights. Each share of Series B Preferred Stock may be converted at any time, at the option of the holder, into 0.0197 shares of our common stock (which is equivalent to a conversion price of $50,760.00 per share) plus cash in lieu of fractional shares. The conversion rate is subject to adjustment upon the occurrence of certain events, as described in the Series B Certificate of Designation. The conversion rate is not adjusted for accumulated and unpaid dividends. If converted, holders of Series B Preferred Stock do not receive a cash payment for all accumulated and unpaid dividends; rather, all accumulated and unpaid dividends are canceled.

We may, at our option, cause shares of Series B Preferred Stock to be automatically converted into that number of shares of our common stock that are issuable at the then-prevailing conversion rate. We may exercise our conversion right only if the closing price of our common stock exceeds 150% of the then-prevailing conversion price ($50,760.00 per share as of April 30, 2026) for 20 trading days during any consecutive 30 trading day period, as described in the Series B Certificate of Designation.

If the holders of Series B Preferred Stock elect to convert their shares in connection with certain “fundamental changes” (as defined in the Series B Certificate of Designation and described below), we will in certain circumstances increase the conversion rate by a number of additional shares of common stock upon conversion or, in lieu thereof, we may in certain circumstances elect to adjust the conversion rate and related conversion obligation so that shares of Series B Preferred Stock are converted into shares of the acquiring or surviving company, in each case as described in the Series B Certificate of Designation.

The adjustment of the conversion price is to prevent dilution of the interests of the holders of the Series B Preferred Stock from certain dilutive transactions with holders of our common stock.

Redemption. We do not have the option to redeem the Series B Preferred Stock. However, holders of the Series B Preferred Stock can require us to redeem all or a portion of their shares of Series B Preferred Stock at a redemption price equal to the Liquidation Preference of the shares to be redeemed in the case of a “fundamental change” (as further described in the Series B Certificate of Designation). A fundamental change will be deemed to have occurred if any of the following occurs:

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any “person” or “group” is or becomes the beneficial owner, directly or indirectly, of 50% or more of the total voting power of all classes of our capital stock then outstanding and normally entitled to vote in the election of directors;

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during any period of two consecutive years, individuals who at the beginning of such period constituted the board of directors of the Company (together with any new directors whose election to our board of directors or whose nomination for election by the stockholders was approved by a vote of 662∕3% of our directors then still in office who were either directors at the beginning of such period or whose election or nomination for election was previously so approved) cease for any reason to constitute a majority of the directors of the Company then in office;

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the termination of trading of our common stock on The Nasdaq Stock Market and our common stock is not approved for trading or quoted on any other U.S. securities exchange or established over-the-counter trading market in the U.S.; or

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we (i) consolidate with or merge with or into another person or another person merges with or into our Company or (ii) sell, assign, transfer, lease, convey or otherwise dispose of all or substantially all of the assets of the Company and certain of its subsidiaries, taken as a whole, to another person and, in the case of any such merger or consolidation described in clause (i), the securities that are outstanding immediately prior to such transaction (and which represent 100% of the aggregate voting power of our voting stock) are changed into or exchanged for cash, securities or property, unless pursuant to the transaction such securities are changed into or exchanged for securities of the surviving person that represent, immediately after such transaction, at least a majority of the aggregate voting power of the voting stock of the surviving person.

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Notwithstanding the foregoing, holders of shares of Series B Preferred Stock will not have the right to require us to redeem their shares if:

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the last reported sale price of shares of our common stock for any five trading days within the 10 consecutive trading days ending immediately before the later of the fundamental change or its announcement equaled or exceeded 105% of the conversion price of the Series B Preferred Stock immediately before the fundamental change or announcement;

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at least 90% of the consideration (excluding cash payments for fractional shares and in respect of dissenters’ appraisal rights) in the transaction or transactions constituting the fundamental change consists of shares of capital stock traded on a U.S. national securities exchange or quoted on The Nasdaq Stock Market, or which will be so traded or quoted when issued or exchanged in connection with a fundamental change, and as a result of the transaction or transactions, shares of Series B Preferred Stock become convertible into such publicly traded securities; or

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in the case of a merger or consolidation constituting a fundamental change (as described in the fourth bullet above), the transaction is effected solely to change our jurisdiction of incorporation.

Moreover, we will not be required to redeem any Series B Preferred Stock upon the occurrence of a fundamental change if a third party makes an offer to purchase the Series B Preferred Stock in the manner, at the price, at the times and otherwise in compliance with the requirements set forth above and such third party purchases all shares of Series B Preferred Stock validly tendered and not withdrawn.

We may, at our option, elect to pay the redemption price in cash, in shares of our common stock valued at a discount of 5% from the market price of shares of our common stock, or in any combination thereof. Notwithstanding the foregoing, we may only pay such redemption price in shares of our common stock that are registered under the Securities Act and eligible for immediate sale in the public market by non-affiliates of our Company.

Voting Rights. Holders of Series B Preferred Stock currently have no voting rights; however, holders may receive certain voting rights, as described in the Series B Certificate of Designation, if (a) dividends on any shares of Series B Preferred Stock, or any other class or series of stock ranking on parity with the Series B Preferred Stock with respect to the payment of dividends, shall be in arrears for dividend periods, whether or not consecutive, containing in the aggregate a number of days equivalent to six calendar quarters or (b) we fail to pay the redemption price, plus accrued and unpaid dividends, if any, on the redemption date for shares of Series B Preferred Stock following a fundamental change. In each such event, the holders of Series B Preferred Stock (voting separately as a class with all other classes or series of stock ranking on parity with the Series B Preferred Stock with respect to the payment of dividends and upon which like voting rights have been conferred and are exercisable) will be entitled to elect two directors to the Company’s board of directors in addition to those directors already serving on the Company’s board of directors at such time (the “Series B Directors”), at the next annual meeting of the Company’s stockholders (or at a special meeting of the Company’s stockholders called for such purpose, whichever is earlier). The right to elect the Series B Directors will continue for each subsequent annual meeting of the Company’s stockholders until all dividends accumulated on the shares of Series B Preferred Stock have been fully paid or set aside for payment or the Company pays in full or sets aside for payment such redemption price, plus accrued but unpaid dividends, if any, on the redemption date for the shares of Series B Preferred Stock following a fundamental change. The term of office of any Series B Directors will terminate immediately upon the termination of the right of holders of Series B Preferred Stock to elect such Series B Directors, as described in this paragraph. Each holder of Series B Preferred Stock will have one vote for each share of Series B Preferred Stock held in the election of Series B Directors. We previously failed to make timely payment of the accrued dividends on the Series B Preferred Stock with respect to the May 15, 2019 and August 15, 2019 dividend payment dates. Such amounts were fully paid on or about November 15, 2019.

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or repeal provisions of our Certificate of Incorporation, including the Series B Certificate of Designation, whether by merger, consolidation or otherwise, so as to adversely amend, alter or affect any power, preference or special right of the outstanding shares of Series B Preferred Stock or the holders thereof without the affirmative vote of not less than two-thirds of the issued and outstanding shares of Series B Preferred Stock.

Warrants and Related Registration Rights

On June 22, 2026, in connection with the CEPA described above, we issued warrants to purchase up to 12,000,000 shares of our common stock to Fit, comprised of three equal tranches of warrants, each to purchase up to 4,000,000 shares of our common stock. The exercise price of these warrants is $26.44 per share, and the warrants are subject to performance-based vesting tied to non-refundable deposits by Fit pursuant to the CEPA. Any warrant that has not vested within 24 months of the date of issuance will automatically terminate and be cancelled. Once a tranche of warrants has vested, that tranche of warrants will expire if not exercised within 24 months following the applicable vesting date for such tranche.

The holders of the warrants are entitled to certain resale shelf registration rights pursuant to a registration rights agreement, dated June 22, 2026, between us and Fit, with respect to the shares of common stock issuable upon exercise of the warrants held by Fit from time to time. Pursuant to the registration rights agreement, the Company is required to file a registration statement under the Securities Act providing for the resale of all or a part of the common stock issuable upon exercise of the warrants held by Fit within 30 calendar days following the closing of the transactions under the CEPA.

Certain Provisions of Delaware Law, Our Certificate of Incorporation, and By-laws

The following description of certain provisions of Delaware law is only a summary. For a complete description, we refer you to the DGCL and to our Certificate of Incorporation and our By-laws, the forms of which are filed as exhibits to the registration statement of which this prospectus forms a part.

Anti-Takeover Provisions

Provisions of our Certificate of Incorporation and By-laws. A number of provisions of our Certificate of Incorporation and By-laws concern matters of corporate governance and the rights of stockholders. Some of these provisions, including, but not limited to, the inability of stockholders to take action by unanimous written consent, certain advance notice requirements for stockholder proposals and director nominations, supermajority voting provisions requiring the affirmative vote of eighty percent of the votes entitled to be cast by the stockholders to amend or repeal voting rights provisions (e.g., provisions giving holders of our common stock one vote per share, provisions prohibiting stockholders from taking action without a meeting, and the supermajority voting provisions), the filling of vacancies on the board of directors by the affirmative vote of a majority of the remaining directors, and the ability of the board of directors to issue shares of preferred stock and to set the voting rights, preferences and other terms thereof without further stockholder action, may be deemed to have an anti-takeover effect and may discourage takeover attempts not first approved by the board of directors, including takeovers which stockholders may deem to be in their best interests. If takeover attempts are discouraged, temporary fluctuations in the market price of shares of our common stock, which may result from actual or rumored takeover attempts, may be inhibited. These provisions, together with the ability of the board of directors to issue preferred stock without further stockholder action, could also delay or frustrate the removal of incumbent directors or the assumption of control by our stockholders, even if the removal or assumption would be beneficial to our stockholders. These provisions could also discourage or inhibit a merger, tender offer or proxy contest, even if favorable to the interests of stockholders, and could depress the market price of our common stock. The board of directors believes these provisions are appropriate to protect our interests and the interests of our stockholders. The board of directors has no present plans to adopt any further measures or devices which may be deemed to have an “anti-takeover effect.”

Delaware Anti-Takeover Provisions. We are subject to Section 203 of the DGCL, which prohibits a publicly-held Delaware corporation from engaging in a “business combination,” except under certain circumstances, with an “interested stockholder” for a period of three years following the date such person became an “interested stockholder” unless:

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before such person became an interested stockholder, the board of directors of the corporation approved either the business combination or the transaction that resulted in the interested stockholder becoming an interested stockholder;

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upon the consummation of the transaction that resulted in the interested stockholder becoming an interested stockholder, the interested stockholder owned at least 85% of the voting stock of the corporation outstanding at the time the transaction commenced, excluding shares held by directors who are also officers of the corporation and shares held by employee stock plans in which employee participants do not have the right to determine confidentially whether shares held subject to the plan will be tendered in a tender or exchange offer; or

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at or following the time such person became an interested stockholder, the business combination is approved by the board of directors of the corporation and authorized at an annual or special meeting of stockholders (and not by written consent) by the affirmative vote of the holders of at least 662∕3% of the outstanding voting stock of the corporation which is not owned by the interested stockholder.

The term “interested stockholder” generally is defined as an entity or person who beneficially owns, or, within the three years prior to the determination of interested stockholder status, owned 15% or more of a corporation’s outstanding voting stock and any entity or person affiliated with or controlling or controlled by the entity or person. The term “business combination” includes mergers, asset or stock sales and other similar transactions resulting in a financial benefit to an interested stockholder. Section 203 makes it more difficult for an “interested stockholder” to effect various business combinations with a corporation for a three-year period. The existence of this provision would be expected to have an anti-takeover effect with respect to transactions not approved in advance by the board of directors, including discouraging attempts that might result in a premium over the market price for the shares of our common stock held by stockholders. A Delaware corporation may “opt out” of Section 203 with an express provision in its original certificate of incorporation or any amendment thereto. Our Certificate of Incorporation does not contain any such exclusion.

Exclusive Forum

Our By-laws provide that unless we consent in writing to an alternative forum, a state court located within the State of Delaware (or, if no state court located within the State of Delaware has jurisdiction, the federal district court for the District of Delaware) will be the sole and exclusive forum for: (i) any derivative action or proceeding brought on behalf of the Company; (ii) any action asserting a claim of breach of a fiduciary duty owed by any of the Company’s directors or officers or other employees to the Company or our stockholders; (iii) any action asserting a claim against the Company or any of our directors or officers or other employees arising pursuant to any provision of the DGCL or our Certificate of Incorporation or our By-laws (as either may be amended from time to time); or (iv) any action asserting a claim against the Company or any of our directors or officers or other employees which claim is governed by the internal affairs doctrine.

Limitations of Directors’ Liability

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for any breach of the director’s duty of loyalty to the Company or our stockholders;

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for acts or omissions not in good faith or which involve intentional misconduct or a knowing violation of law;

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under Section 174 of the DGCL; or

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for any transaction from which the director derived an improper personal benefit.

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fiduciary duty as a director (including breaches resulting from grossly negligent behavior), except in the situations described above. These provisions do not limit the liability of directors under federal securities laws and do not affect the availability of equitable remedies such as an injunction or rescission based upon a director’s breach of his duty of care.

Disclosure of Commission Position on Indemnification for Securities Act Liabilities

Our Certificate of Incorporation provides that none of our directors will be personally liable to the Company or our stockholders for monetary damages for breach of fiduciary duty as a director, except to the extent described above. Our By-laws provide for indemnification of our officers and directors to the fullest extent permitted by applicable law. Insofar as indemnification for liabilities under the Securities Act may be permitted to directors, officers or controlling persons of the Company pursuant to our Certificate of Incorporation, our By-laws, applicable law, or otherwise, we have been informed that in the opinion of the SEC such indemnification is against public policy as expressed in the Securities Act and is therefore unenforceable.

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MATERIAL UNITED STATES FEDERAL INCOME TAX CONSIDERATIONS

The following is a discussion of material United States federal income tax consequences of the ownership and disposition of our common stock. This discussion applies only to shares of common stock that are held as capital assets for U.S. federal income tax purposes (generally, property held for investment) and is applicable only to persons who are receiving our common stock in this offering.

This discussion is a summary only and does not describe all of the tax consequences that may be relevant to you in light of your particular circumstances, including but not limited to the application of the alternative minimum tax and the Medicare tax on certain investment income and the different consequences that may apply if you are subject to special rules that apply to certain types of investors, including but not limited to:

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financial institutions or financial services entities;

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brokers or dealers in securities;

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governments or agencies or instrumentalities thereof;

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regulated investment companies and real estate investment trusts;

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S corporations and any beneficial owners of such entities;

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corporations that accumulate earnings to avoid U.S. federal income tax;

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certain U.S. expatriates, former U.S. citizens or former long-term residents of the United States;

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persons that actually or constructively own five percent (5%) or more (by vote or value) of our common stock;

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insurance companies;

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dealers or traders subject to a mark-to-market method of tax accounting with respect to our common stock;

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accrual-method taxpayers who are required under Section 451(b) of the Internal Revenue Code of 1986, as amended to the date hereof (the “Code”), to recognize income for U.S. federal income tax purposes no later than when such income is taken into account in an applicable financial statement;

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persons holding the common stock as part of a straddle, hedge, conversion transaction, synthetic security or other integrated or similar transaction;

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persons deemed to sell our common stock under the constructive sale provisions of the Code;

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U.S. holders (as defined below) whose functional currency is not the U.S. dollar;

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“qualified foreign pension funds” as defined in Section 897(l)(2) of the Code and entities all of the interests of which are held by qualified foreign pension funds;

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partnerships or other pass-through entities for U.S. federal income tax purposes and any beneficial owners of such entities;

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“controlled foreign corporations” and “passive foreign investment companies”;

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persons who acquire our common stock as compensation; and

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tax-exempt entities.

If a partnership (including an entity or arrangement treated as a partnership for U.S. federal income tax purposes) or other pass-through entity holds our common stock, the U.S. federal income tax treatment of a partner in such partnership or equityholder in such pass-through entity generally will depend upon the status of the partner or equityholder, upon the activities of the partnership or other pass-through entity and upon certain determinations made at the partner or equityholder level. Accordingly, we urge partners in partnerships (including entities or arrangements treated as partnerships for U.S. federal income tax purposes) and equityholders in other pass-through entities considering the acquisition of our common stock

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to consult their tax advisors regarding the U.S. federal income tax considerations of the ownership and disposition of our common stock by such partnership or pass-through entity.

This discussion is based on the Code, and administrative pronouncements, judicial decisions and final, temporary and proposed regulations promulgated by the United States Department of the Treasury (the “Treasury Regulations”) as of the date hereof, which are subject to change, possibly on a retroactive basis, and changes to any of which subsequent to the date of this prospectus supplement may affect the tax consequences described herein. This discussion does not address any aspect of state, local or non-U.S. taxation, or any U.S. federal taxes other than income taxes (such as gift and estate taxes). We have not sought, and will not seek, a ruling from the Internal Revenue Service (the “IRS”) as to any U.S. federal income tax consequence described herein. The IRS may disagree with the discussion herein, and its determination may be upheld by a court. Moreover, there can be no assurance that future legislation, Treasury Regulations, administrative rulings or court decisions will not adversely affect the accuracy of the statements in this discussion.

THIS DISCUSSION OF U.S. FEDERAL INCOME TAX CONSIDERATIONS IS FOR GENERAL INFORMATION PURPOSES ONLY AND IS NOT TAX ADVICE. WE URGE PROSPECTIVE HOLDERS TO CONSULT THEIR TAX ADVISORS CONCERNING THE U.S. FEDERAL INCOME TAX CONSEQUENCES TO THEM OF OWNING AND DISPOSING OF OUR COMMON STOCK, AS WELL AS THE APPLICATION OF ANY STATE, LOCAL AND NON-U.S. INCOME, ESTATE AND OTHER TAX CONSIDERATIONS.

U.S. Holders

This section applies to you if you are a “U.S. holder.” As used herein, the term “U.S. holder” means a beneficial owner of our common stock who or that is for U.S. federal income tax purposes:

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an individual who is a citizen or resident of the United States;

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a corporation (or other entity taxable as a corporation) organized in or under the laws of the United States, any state thereof or the District of Columbia;

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an estate the income of which is subject to U.S. federal income taxation regardless of its source; or

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a trust, if (A) a U.S. court is able to exercise primary supervision over the trust’s administration and one or more U.S. persons have authority to control all of the trust’s substantial decisions or (B) the trust has validly elected to be treated as a U.S. person for U.S. federal income tax purposes.

Taxation of Distributions

If we pay distributions in cash or other property (other than certain distributions of our stock or rights to acquire our stock) to U.S. holders of shares of our common stock, such distributions generally will constitute dividends for U.S. federal income tax purposes to the extent paid from our current or accumulated earnings and profits, as determined under U.S. federal income tax principles. Distributions in excess of our current and accumulated earnings and profits will constitute a return of capital that will be applied against and reduce (but not below zero) the U.S. holder’s adjusted tax basis in our common stock. Any remaining excess will be treated as gain realized on the sale or other disposition of the common stock and will be treated as described below under “—Gain or Loss on Sale, Taxable Exchange or Other Taxable Disposition of Common Stock.” Dividends we pay to a U.S. holder that is a taxable corporation generally will qualify for the dividends received deduction if the requirements relating to the requisite holding period are satisfied. With certain exceptions, and provided certain holding period requirements are met, dividends we pay to a non-corporate U.S. holder generally will constitute “qualified dividends” that currently are subject to tax at preferential long-term capital gains rates.

Gain or Loss on Sale, Taxable Exchange or Other Taxable Disposition of Common Stock

Upon a sale or other taxable disposition of our common stock, a U.S. holder generally will recognize capital gain or loss in an amount equal to the difference between (i) the amount realized, which is the sum of the amount of cash and the fair market value of any property received in such disposition and (ii) the U.S. holder’s adjusted tax basis in the common stock transferred in such disposition. Any such capital gain or

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loss generally will be long-term capital gain or loss if the U.S. holder’s holding period for the common stock exceeds one year at the time of the sale or other taxable disposition. Long-term capital gains recognized by non-corporate U.S. holders currently are eligible to be taxed at reduced rates. The deductibility of capital losses is subject to limitations.

Information Reporting and Backup Withholding

In general, information reporting requirements may apply to dividends paid to a U.S. holder and to the proceeds of the sale or other disposition of our common stock, unless the U.S. holder is an exempt recipient. Backup withholding may apply to such payments if the U.S. holder fails to provide a taxpayer identification number or a certification of exempt status or has been notified by the IRS that it is subject to backup withholding (and such notification has not been withdrawn).

Backup withholding is not an additional tax. Any amounts withheld under the backup withholding rules will be allowed as a refund or a credit against a U.S. holder’s U.S. federal income tax liability provided the required information is timely furnished to the IRS.

All U.S. holders should consult their tax advisors regarding the application of information reporting and backup withholding to them.

Non-U.S. Holders

This section applies to you if you are a “Non-U.S. holder.” As used herein, the term “Non-U.S. holder” means a beneficial owner of our common stock who or that is for U.S. federal income tax purposes:

•

a non-resident alien individual (other than certain former citizens and residents of the United States subject to U.S. tax as expatriates);

•

a foreign corporation or any other foreign association taxable as a corporation for U.S. federal income tax purposes; or

•

an estate or trust that is not a U.S. holder;

but generally does not include an individual who is present in the United States for 183 days or more in the taxable year of disposition. If you are such an individual, you should consult your tax advisor regarding the U.S. federal income tax consequences of the acquisition, ownership or sale or other disposition of our common stock.

Taxation of Distributions

In general, any distributions (other than certain distributions of our stock or rights to acquire our stock) made to a Non-U.S. holder of shares of our common stock, to the extent paid out of our current or accumulated earnings and profits (as determined under U.S. federal income tax principles), will constitute dividends for U.S. federal income tax purposes. Provided such dividends are not effectively connected with the Non-U.S. holder’s conduct of a trade or business within the United States (and, under certain income tax treaties, not attributable to a U.S. permanent establishment or fixed base maintained by the Non-U.S. holder), we will be required to withhold tax from the gross amount of the dividend at a rate of thirty percent (30%), unless such Non-U.S. holder is eligible for a reduced rate of withholding tax under an applicable income tax treaty and provides proper certification of its eligibility for such reduced rate (usually on an IRS Form W-8BEN or W-8BEN-E). Any distribution not constituting a dividend will be treated first as reducing (but not below zero) the Non-U.S. holder’s adjusted tax basis in its common stock and, to the extent such distribution exceeds the Non-U.S. holder’s adjusted tax basis, as gain realized from the sale or other disposition of our common stock, which will be treated as described below under “Non-U.S. Holders—Gain or Loss on Sale, Taxable Exchange or Other Taxable Disposition of Common Stock” below.

This withholding tax generally does not apply to dividends paid to a Non-U.S. holder who provides a Form W-8ECI certifying that the dividends are effectively connected with the Non-U.S. holder’s conduct of a trade or business within the United States. Instead, the effectively connected dividends will be subject to regular U.S. federal income tax as if the Non-U.S. holder were a U.S. resident, subject to an applicable income

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tax treaty providing otherwise. A corporate Non-U.S. holder receiving effectively connected dividends may also be subject to an additional “branch profits tax” imposed at a rate of 30% (or a lower applicable treaty rate).

Gain or Loss on Sale, Taxable Exchange or Other Taxable Disposition of Common Stock

A Non-U.S. holder generally will not be subject to U.S. federal income or withholding tax in respect of gain recognized on a sale, taxable exchange or other taxable disposition of our common stock, unless:

•

the gain is effectively connected with the conduct by the Non-U.S. holder of a trade or business within the United States (and, under certain income tax treaties, is attributable to a U.S. permanent establishment or fixed base maintained by the Non-U.S. holder);

•

such Non-U.S. holder is a non-resident alien individual who was present in the United States for 183 days or more in the taxable year of such disposition and certain other requirements are met; or

•

we are or have been a “United States real property holding corporation” for U.S. federal income tax purposes at any time during the shorter of the five-year period ending on the date of disposition or the period that the Non-U.S. holder held our common stock and, in the case where shares of our common stock are regularly traded on an established securities market, the Non-U.S. holder has owned, directly or constructively, more than five percent (5%) of our common stock at any time within the shorter of the five-year period preceding the disposition or such Non-U.S. holder’s holding period for the shares of our common stock. There can be no assurance that our common stock will be treated as regularly traded on an established securities market for this purpose during your ownership thereof.

Unless an applicable treaty provides otherwise, gain described in the first bullet point above will be subject to tax at generally applicable U.S. federal income tax rates as if the Non-U.S. holder were a U.S. resident. Any gains described in the first bullet point above of a corporate Non-U.S. holder may also be subject to an additional “branch profits tax” at a thirty percent (30%) rate (or a lower applicable income tax treaty rate). If the second bullet point applies to a Non-U.S. holder, such Non-U.S. holder will be subject to U.S. tax on such Non-U.S. holder’s U.S. source net capital gain for such year (which may be reduced by certain U.S. source capital losses) at a tax rate of thirty percent (30%) (or a lower applicable income tax treaty rate).

If the third bullet point above applies to a Non-U.S. holder, gain recognized by such holder will be subject to tax at generally applicable U.S. federal income tax rates applicable to a U.S. holder. In addition, a buyer may be required to withhold U.S. federal income tax at a rate of fifteen percent (15%) of the amount realized upon such disposition. Generally, a corporation is a United States real property holding corporation only if the fair market value of its U.S. real property interest equals or exceeds 50% of the sum of the fair market value of its worldwide real property interest plus its other assts used or held for use in a trade or business. We believe that we are not, and do not anticipate becoming, a United States real property holding corporation. However, such determination is factual in nature and subject to change and no assurance can be provided as to whether we would be treated as a United States real property holding corporation in any future year.

Information Reporting and Backup Withholding

Information returns will be filed with the IRS in connection with payments of dividends and may be filed in connection with the payment of proceeds from a sale or other disposition of our common stock. A Non-U.S. holder may have to comply with certification procedures to establish that it is not a United States person in order to avoid information reporting and backup withholding requirements. The certification procedures required to claim a reduced rate of withholding under a treaty generally will satisfy the certification requirements necessary to avoid backup withholding as well.

Backup withholding is not an additional tax. The amount of any backup withholding from a payment to a Non-U.S. holder will be allowed as a credit against such holder’s U.S. federal income tax liability and may entitle such holder to a refund, provided that the required information is timely furnished to the IRS.

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All Non-U.S. holders should consult their tax advisors regarding the application of information reporting and backup withholding to them.

FATCA Withholding Taxes

Sections 1471 through 1474 of the Code and the Treasury Regulations and administrative guidance promulgated thereunder (commonly referred to as FATCA) generally impose withholding at a rate of thirty percent (30%) in certain circumstances on dividends in respect of our common stock which are held by or through certain foreign financial institutions (including investment funds), unless any such institution (1) enters into, and complies with, an agreement with the IRS to report, on an annual basis, information with respect to interests in, and accounts maintained by, the institution that are owned by certain U.S. persons and by certain non-U.S. entities that are wholly or partially owned by U.S. persons and to withhold on certain payments, or (2) if required under an intergovernmental agreement between the United States and an applicable foreign country, reports such information to its local tax authority, which will exchange such information with the U.S. authorities. An intergovernmental agreement between the United States and an applicable foreign country may modify these requirements. Accordingly, the entity through which our common stock is held will affect the determination of whether such withholding is required. Similarly, dividends in respect of our common stock held by an investor that is a non-financial non-U.S. entity that does not qualify under certain exceptions will generally be subject to withholding at a rate of thirty percent (30%), unless such entity either (1) certifies to us or the applicable withholding agent that such entity does not have any “substantial United States owners” or (2) provides certain information regarding the entity’s “substantial United States owners,” which will in turn be provided to the U.S. Department of the Treasury. All holders of common stock should consult their tax advisors regarding the possible implications of FATCA on their ownership of our common stock.

The preceding discussion of U.S. federal income tax considerations is for general information only. It is not tax advice. Each prospective investor should consult its tax advisor regarding the particular U.S. federal, state and local and non-U.S. tax consequences of holding and disposing of our common stock, including the consequences of any proposed change in applicable laws.

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UNDERWRITING

Citigroup Global Markets Inc. and Barclays Capital Inc. are acting as joint book-running managers of the offering and as representatives of the underwriters named below. Subject to the terms and conditions stated in the underwriting agreement dated the date of this prospectus supplement, each underwriter named below has severally agreed to purchase, and we have agreed to sell to that underwriter, the number of shares set forth opposite the underwriter’s name.

Underwriter	Number of Shares
Citigroup Global Markets Inc.		4,071,429
Barclays Capital Inc.		3,214,286
Oppenheimer & Co. Inc.		1,071,429
RBC Capital Markets, LLC		428,571
Goldman Sachs & Co. LLC		428,571
Canaccord Genuity LLC		428,571
B. Riley Securities, Inc.		321,429
BMO Capital Markets Corp.		321,429
Siebert Williams Shank & Co., LLC		321,429
Tuohy Brothers Investment Research, Inc.		107,142
Total		10,714,286

The underwriting agreement provides that the obligations of the underwriters to purchase the shares included in this offering are subject to approval of legal matters by counsel and to other conditions. The underwriters are obligated to purchase all the shares (other than those covered by the underwriters’ option described below) if they purchase any of the shares.

Shares sold by the underwriters to the public will initially be offered at the initial public offering price set forth on the cover of this prospectus supplement. Any shares sold by the underwriters to securities dealers may be sold at a discount from the initial public offering price not to exceed $0.63 per share. If all the shares are not sold at the initial offering price, the underwriters may change the offering price and the other selling terms.

We have granted to the underwriters an option, exercisable for 30 days from the date of this prospectus supplement, to purchase up to 1,607,143 additional shares at the public offering price less the underwriting discount. To the extent the option is exercised, each underwriter must purchase a number of additional shares approximately proportionate to that underwriter’s initial purchase commitment. Any shares issued or sold under the option will be issued and sold on the same terms and conditions as the other shares that are the subject of this offering.

We, our officers and our directors have agreed that, for a period of 60 days from the date of this prospectus supplement, subject to certain exceptions, we and they will not, without the prior written consent of Citigroup Global Markets Inc. and Barclays Capital Inc., offer, sell, contract to sell, pledge or otherwise dispose of, directly or indirectly, any shares of or any securities convertible into or exercisable or exchangeable for our common stock. Citigroup Global Markets Inc. and Barclays Capital Inc., in their sole discretion, may release any of the securities subject to these lock-up agreements at any time without notice.

The shares are listed on The Nasdaq Global Market under the symbol “FCEL.”

The following table shows the underwriting discounts and commissions that we are to pay to the underwriters in connection with this offering. These amounts are shown assuming both no exercise and full exercise of the underwriters’ option.

	No Exercise			Full Exercise
Per share		$	1.05		$	1.05
Total		$	11,250,000		$	12,937,500

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We estimate that the total expenses of this offering, including registration, filing and listing fees, printing fees and legal and accounting expenses, but excluding the underwriting discounts and commissions, will be approximately $400,000. We have agreed to reimburse the underwriters for certain of their expenses.

In connection with the offering, the underwriters may purchase and sell shares in the open market. Purchases and sales in the open market may include short sales, purchases to cover short positions, which may include purchases pursuant to the option, and stabilizing purchases.

•

Short sales involve secondary market sales by the underwriters of a greater number of shares than they are required to purchase in the offering.

•

“Covered” short sales are sales of shares in an amount up to the number of shares represented by the underwriters’ option.

•

“Naked” short sales are sales of shares in an amount in excess of the number of shares represented by the underwriters’ option.

•

Covering transactions involve purchases of shares either pursuant to the underwriters’ option or in the open market after the distribution has been completed in order to cover short positions.

•

To close a naked short position, the underwriters must purchase shares in the open market after the distribution has been completed. A naked short position is more likely to be created if the underwriters are concerned that there may be downward pressure on the price of the shares in the open market after pricing that could adversely affect investors who purchase in the offering.

•

To close a covered short position, the underwriters must purchase shares in the open market after the distribution has been completed or must exercise the underwriters’ option. In determining the source of shares to close the covered short position, the underwriters will consider, among other things, the price of shares available for purchase in the open market as compared to the price at which they may purchase shares through the underwriters’ option.

•

Stabilizing transactions involve bids to purchase shares so long as the stabilizing bids do not exceed a specified maximum.

Purchases to cover short positions and stabilizing purchases, as well as other purchases by the underwriters for their own accounts, may have the effect of preventing or retarding a decline in the market price of the shares. They may also cause the price of the shares to be higher than the price that would otherwise exist in the open market in the absence of these transactions. The underwriters may conduct these transactions on The Nasdaq Global Market, in the over-the-counter market or otherwise. If the underwriters commence any of these transactions, they may discontinue them at any time.

In connection with the offering, underwriters and selling group members may engage in passive market making transactions in the common stock on The Nasdaq Global Market in accordance with Rule 103 of Regulation M under the Exchange Act during the period before the commencement of offers or sales of common stock and extending through the completion of distribution. A passive market maker must display its bids at a price not in excess of the highest independent bid of the security. However, if all independent bids are lowered below the passive market maker’s bid that bid must be lowered when specified purchase limits are exceeded.

The underwriters and certain of their affiliates have performed commercial banking, investment banking and advisory services for us from time to time for which they have received customary fees and reimbursement of expenses. The underwriters and certain of their affiliates may, from time to time, engage in transactions with and perform services for us in the ordinary course of their business for which they may receive customary fees and reimbursement of expenses. For example, Citigroup Global Markets Inc. and Barclays Capital Inc. and certain of the other underwriters are parties, as sales agents, to the Open Market Sale Agreement, pursuant to which we may offer and sell, from time to time, shares of our common stock through the sales agents in an “at the market offering” as defined in Rule 415(a)(4) promulgated under the Securities Act, and for which the sales agents are entitled to receive customary commissions.

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In the ordinary course of their various business activities, the underwriters and certain of their affiliates may make or hold a broad array of investments and actively trade debt and equity securities (or related derivative securities) and financial instruments (including bank loans) for their own account and for the accounts of their customers, and such investment and securities activities may involve securities and/or instruments of ours or our affiliates. The underwriters and certain of their affiliates may also communicate independent investment recommendations, market color or trading ideas and/or publish or express independent research views in respect of our securities or instruments and may at any time hold, or recommend to clients that they acquire, long and/or short positions in such securities and instruments.

We have agreed to indemnify the underwriters against certain liabilities, including liabilities under the Securities Act, or to contribute to payments the underwriters may be required to make because of any of those liabilities.

A prospectus supplement in electronic format may be made available on the Internet sites or through other online services maintained by one or more of the underwriters and/or selling group members participating in this offering, or by their affiliates. In those cases, prospective investors may view offering terms online and, depending upon the particular underwriter or selling group member, prospective investors may be allowed to place orders online. The underwriters may agree with us to allocate a specific number of shares for sale to online brokerage account holders. Any such allocation for online distributions will be made by the representatives on the same basis as other allocations.

Other than the prospectus supplement in electronic format, the information on any underwriter’s or selling group member’s web site and any information contained in any other web site maintained by an underwriter or selling group member is not part of the prospectus or the registration statement of which this prospectus forms a part, has not been approved and/or endorsed by us or any underwriter or selling group member in its capacity as underwriter or selling group member and should not be relied upon by investors.

If you purchase shares of common stock offered in this prospectus supplement, you may be required to pay stamp taxes and other charges under the laws and practices of the country of purchase, in addition to the offering price listed on the cover page of this prospectus supplement.

Notice to Prospective Investors in the European Economic Area

In relation to each member state of the European Economic Area that has implemented the Prospectus Directive (each, a relevant member state), with effect from and including the date on which the Prospectus Directive is implemented in that relevant member state (the relevant implementation date), an offer of shares described in this prospectus supplement may not be made to the public in that relevant member state prior to the publication of a prospectus in relation to the shares that has been approved by the competent authority in that relevant member state or, where appropriate, approved in another relevant member state and notified to the competent authority in that relevant member state, all in accordance with the Prospectus Directive, except that, with effect from and including the relevant implementation date, an offer of securities may be offered to the public in that relevant member state at any time:

•

to any legal entity that is authorized or regulated to operate in the financial markets or, if not so authorized or regulated, whose corporate purpose is solely to invest in securities;

•

to any legal entity that has two or more of (1) an average of at least 250 employees during the last financial year; (2) a total balance sheet of more than €43,000,000 and (3) an annual net turnover of more than €50,000,000, as shown in its last annual or consolidated accounts;

•

to fewer than 100 natural or legal persons (other than qualified investors as defined below) subject to obtaining the prior consent of the representatives for any such offer; or

•

in any other circumstances that do not require the publication of a prospectus pursuant to Article 3 of the Prospectus Directive.

Each purchaser of shares described in this prospectus supplement located within a relevant member state will be deemed to have represented, acknowledged and agreed that it is a “qualified investor” within the meaning of Article 2(1)(e) of the Prospectus Directive.

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For purposes of this provision, the expression an “offer to the public” in any relevant member state means the communication in any form and by any means of sufficient information on the terms of the offer and the securities to be offered so as to enable an investor to decide to purchase or subscribe the securities, as the expression may be varied in that member state by any measure implementing the Prospectus Directive in that member state, and the expression “Prospectus Directive” means Directive 2003/71/EC and includes any relevant implementing measure in each relevant member state.

The sellers of the shares have not authorized and do not authorize the making of any offer of shares through any financial intermediary on their behalf, other than offers made by the underwriters with a view to the final placement of the shares as contemplated in this prospectus supplement. Accordingly, no purchaser of the shares, other than the underwriters, is authorized to make any further offer of the shares on behalf of the sellers or the underwriters.

Notice to Prospective Investors in the United Kingdom

This prospectus supplement and the accompanying prospectus are only being distributed to, and is only directed at, persons in the United Kingdom that are qualified investors within the meaning of Article 2(1)(e) of the Prospectus Directive that are also (i) investment professionals falling within Article 19(5) of the Financial Services and Markets Act 2000 (Financial Promotion) Order 2005 (the “Order”) or (ii) high net worth entities, and other persons to whom it may lawfully be communicated, falling within Article 49(2)(a) to (d) of the Order (each such person being referred to as a “relevant person”). This prospectus supplement and its contents are confidential and should not be distributed, published or reproduced (in whole or in part) or disclosed by recipients to any other persons in the United Kingdom. Any person in the United Kingdom that is not a relevant person should not act or rely on this document or any of its contents.

Notice to Prospective Investors in France

Neither this prospectus supplement nor any other offering material relating to the shares described in this prospectus supplement has been submitted to the clearance procedures of the Autorité des Marchés Financiers or of the competent authority of another member state of the European Economic Area and notified to the Autorité des Marchés Financiers. The shares have not been offered or sold and will not be offered or sold, directly or indirectly, to the public in France. Neither this prospectus supplement nor any other offering material relating to the shares has been or will be:

•

released, issued, distributed or caused to be released, issued or distributed to the public in France; or

•

used in connection with any offer for subscription or sale of the shares to the public in France.

Such offers, sales and distributions will be made in France only:

•

to qualified investors (investisseurs qualifiés) and/or to a restricted circle of investors (cercle restreint d’investisseurs), in each case investing for their own account, all as defined in, and in accordance with articles L.411-2, D.411-1, D.411-2, D.734-1, D.744-1, D.754-1 and D.764-1 of the French Code monétaire et financier;

•

to investment services providers authorized to engage in portfolio management on behalf of third parties; or

•

in a transaction that, in accordance with article L.411-2-II-1°-or-2°-or 3° of the French Code monétaire et financier and article 211-2 of the General Regulations (Règlement Général) of the Autorité des Marchés Financiers, does not constitute a public offer (appel public à l’épargne).

The shares may be resold directly or indirectly, only in compliance with articles L.411-1, L.411-2, L.412-1 and L.621-8 through L.621-8-3 of the French Code monétaire et financier.

Notice to Prospective Investors in Hong Kong

The shares may not be offered or sold in Hong Kong by means of any document other than (i) in circumstances which do not constitute an offer to the public within the meaning of the Companies Ordinance

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(Cap. 32, Laws of Hong Kong), or (ii) to “professional investors” within the meaning of the Securities and Futures Ordinance (Cap. 571, Laws of Hong Kong) and any rules made thereunder, or (iii) in other circumstances which do not result in the document being a “prospectus” within the meaning of the Companies Ordinance (Cap. 32, Laws of Hong Kong) and no advertisement, invitation or document relating to the shares may be issued or may be in the possession of any person for the purpose of issue (in each case whether in Hong Kong or elsewhere), which is directed at, or the contents of which are likely to be accessed or read by, the public in Hong Kong (except if permitted to do so under the laws of Hong Kong) other than with respect to shares which are or are intended to be disposed of only to persons outside Hong Kong or only to “professional investors” within the meaning of the Securities and Futures Ordinance (Cap. 571, Laws of Hong Kong) and any rules made thereunder.

Notice to Prospective Investors in Japan

The shares offered in this prospectus supplement have not been registered under the Securities and Exchange Law of Japan. The shares have not been offered or sold and will not be offered or sold, directly or indirectly, in Japan or to or for the account of any resident of Japan, except (i) pursuant to an exemption from the registration requirements of the Securities and Exchange Law and (ii) in compliance with any other applicable requirements of Japanese law.

Notice to Prospective Investors in Singapore

This prospectus supplement has not been registered as a prospectus with the Monetary Authority of Singapore. Accordingly, this prospectus supplement and any other document or material in connection with the offer or sale, or invitation for subscription or purchase, of the shares may not be circulated or distributed, nor may the shares be offered or sold, or be made the subject of an invitation for subscription or purchase, whether directly or indirectly, to persons in Singapore other than (i) to an institutional investor under Section 274 of the Securities and Futures Act, Chapter 289 of Singapore (the “SFA”), (ii) to a relevant person pursuant to Section 275(1), or any person pursuant to Section 275(1A), and in accordance with the conditions specified in Section 275 of the SFA or (iii) otherwise pursuant to, and in accordance with the conditions of, any other applicable provision of the SFA, in each case subject to compliance with conditions set forth in the SFA.

Where the shares are subscribed or purchased under Section 275 of the SFA by a relevant person which is:

•

a corporation (which is not an accredited investor (as defined in Section 4A of the SFA)) the sole business of which is to hold investments and the entire share capital of which is owned by one or more individuals, each of whom is an accredited investor; or

•

a trust (where the trustee is not an accredited investor) whose sole purpose is to hold investments and each beneficiary of the trust is an individual who is an accredited investor,

shares, debentures and units of shares and debentures of that corporation or the beneficiaries’ rights and interest (howsoever described) in that trust shall not be transferred within six months after that corporation or that trust has acquired the shares pursuant to an offer made under Section 275 of the SFA except:

•

to an institutional investor (for corporations, under Section 274 of the SFA) or to a relevant person defined in Section 275(2) of the SFA, or to any person pursuant to an offer that is made on terms that such shares, debentures and units of shares and debentures of that corporation or such rights and interest in that trust are acquired at a consideration of not less than S$200,000 (or its equivalent in a foreign currency) for each transaction, whether such amount is to be paid for in cash or by exchange of securities or other assets, and further for corporations, in accordance with the conditions specified in Section 275 of the SFA;

•

where no consideration is or will be given for the transfer; or

•

where the transfer is by operation of law.

Notice to Prospective Investors in Switzerland

The offering of the shares of common stock in Switzerland is exempt from requirement to prepare and publish a prospectus under the Swiss Financial Services Act (“FinSA”) because such offering is made to

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professional clients within the meaning of the FinSA only and the shares of common stock will not be admitted to trading on any trading venue (exchange or multilateral trading facility) in Switzerland. This document does not constitute a prospectus pursuant to the FinSA, and no such prospectus has been or will be prepared for or in connection with the offering of the shares of common stock.

Notice to Prospective Investors in Dubai International Financial Centre (“DIFC”)

This prospectus supplement relates to an exempt offer which is not subject to any form of regulation or approval by the Dubai Financial Services Authority (the “DFSA”). The DFSA has not approved this prospectus supplement nor has any responsibility for reviewing or verifying any document or other documents in connection with the offering. Accordingly, the DFSA has not approved this prospectus supplement or any other associated documents nor taken any steps to verify the information set out in this prospectus supplement, and has no responsibility for it.

The shares of common stock have not been offered and will not be offered to any persons in the DIFC except on the basis that an offer is:

(i) an “Exempt Offer” in accordance with the Markets Rules (MKT) Module of the DFSA Rulebook; and

(ii) made only to persons who meet the “Deemed Professional Client” criteria set out in Rule 2.3.4 of the Conduct of Business (COB) module of the DFSA Rulebook, who are not natural persons.

Notice to Prospective Investors in Canada

The shares of common stock may be sold only to purchasers purchasing, or deemed to be purchasing, as principal that are accredited investors, as defined in National Instrument 45-106 Prospectus Exemptions, and are permitted clients, as defined in National Instrument 31-103 Registration Requirements, Exemptions and Ongoing Registrant Obligations. Any resale of the shares of common stock must be made in accordance with an exemption from, or in a transaction not subject to, the prospectus requirements of applicable securities laws.

Securities legislation in certain provinces or territories of Canada may provide a purchaser with remedies for rescission or damages if this prospectus supplement (including any amendment thereto) contains a misrepresentation, provided that the remedies for rescission or damages are exercised by the purchaser within the time limit prescribed by the securities legislation of the purchaser’s province or territory. The purchaser should refer to any applicable provisions of the securities legislation of the purchaser’s province or territory for particulars of these rights or consult with a legal advisor.

Pursuant to section 3A.3 (or, in the case of securities issued or guaranteed by the government of a non-Canadian jurisdiction, section 3A.4) of National Instrument 33-105 Underwriting Conflicts (NI 33-105), the underwriters are not required to comply with the disclosure requirements of NI 33-105 regarding underwriter conflicts of interest in connection with this offering.

Notice to Prospective Investors in Australia

No placement document, prospectus, product disclosure statement, or other disclosure document has been lodged with the Australian Securities and Investments Commission in relation to the offering. This offering document does not constitute a prospectus, product disclosure statement or other disclosure document under the Corporations Act 2001, or the Corporations Act, and does not purport to include the information required for a prospectus, product disclosure statement or other disclosure document under the Corporations Act.

Any offer in Australia of the shares may only be made to persons, or the Exempt Investors who are “sophisticated investors” (within the meaning of section 708(8) of the Corporations Act), “professional investors” (within the meaning of section 708(11) of the Corporations Act), or otherwise pursuant to one or more exemptions contained in section 708 of the Corporations Act so that it is lawful to offer the shares without disclosure to investors under Chapter 6D of the Corporations Act.

The shares applied for by Exempt Investors in Australia must not be offered for sale in Australia in the period of 12 months after the date of allotment under the offering, except in circumstances where disclosure

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to investors under Chapter 6D of the Corporations Act would not be required pursuant to an exemption under section 708 of the Corporations Act or otherwise or where the offer is pursuant to a disclosure document which complies with Chapter 6D of the Corporations Act. Any person acquiring shares must observe such Australian on-sale restrictions.

This offering document contains general information only and does not take account of the investment objectives, financial situation, or particular needs of any particular person. It does not contain any securities recommendations or financial product advice. Before making an investment decision, investors need to consider whether the information in this offering document is appropriate to their needs, objectives, and circumstances, and, if necessary, seek expert advice on those matters.

LEGAL MATTERS

The validity of the shares of common stock being offered hereby will be passed upon for us by Foley & Lardner LLP. Legal matters in connection with the common stock offered hereby will be passed upon for the underwriters by Duane Morris LLP.

EXPERTS

The consolidated financial statements of FuelCell Energy, Inc. as of October 31, 2025 and 2024, and for each of the years in the three-year period ended October 31, 2025, and management’s assessment of the effectiveness of internal control over financial reporting as of October 31, 2025 have been incorporated by reference herein and in the registration statement in reliance upon the report of KPMG LLP, independent registered public accounting firm, incorporated by reference herein, and upon the authority of said firm as experts in accounting and auditing.

WHERE YOU CAN FIND ADDITIONAL INFORMATION

We file annual, quarterly and current reports, proxy statements and other information with the SEC. We also filed a registration statement on Form S-3, including exhibits, under the Securities Act with respect to the shares of common stock offered by this prospectus supplement. The SEC maintains a website at http://www.sec.gov, which contains reports, proxy and information statements and other information regarding issuers that file electronically with the SEC. Our SEC filings are also available to the public on the “Investors” tab of our website at www.fuelcellenergy.com. The information on, or that can be accessed through, our website is not incorporated by reference in this prospectus supplement or in the accompanying prospectus, and you should not consider it to be a part of this prospectus supplement or the accompanying prospectus. Our web site address is included as an inactive textual reference only.

This prospectus supplement and the accompanying prospectus are part of a registration statement that we have filed with the SEC relating to the securities to be offered. This prospectus supplement and the accompanying prospectus do not contain all of the information we have included in the registration statement and the accompanying exhibits and schedules in accordance with the rules and regulations of the SEC, and we refer you to the omitted information. The statements this prospectus supplement makes pertaining to the content of any contract, agreement or other document that is an exhibit to the registration statement necessarily are summaries of their material provisions and do not describe all provisions, exceptions and qualifications contained in those contracts, agreements or documents. You should read those contracts, agreements and documents for information that may be important to you. The registration statement, exhibits and schedules are available on the SEC’s website.

The SEC allows us to incorporate by reference much of the information we file with the SEC, which means that we can disclose important information to you by referring you to those publicly available documents. The information that we incorporate by reference in this prospectus supplement is considered to be part of this prospectus supplement. Because we are incorporating by reference future filings with the SEC, this prospectus supplement is continually updated and those future filings may modify or supersede some of the information included or incorporated in this prospectus supplement. This means that you must look at all of the SEC filings that we incorporate by reference to determine if any of the statements in this prospectus supplement or in any document previously incorporated by reference have been modified or superseded. This prospectus supplement incorporates by reference the documents listed below and any future filings we make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act (in each

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case, other than those documents or the portions of those documents not deemed to be filed) prior to the completion or termination of the offering:

•

our Annual Report on Form 10-K for the fiscal year ended October 31, 2025 filed with the SEC on December 18, 2025;

•

our Quarterly Report on Form 10-Q for the fiscal quarter ended January 31, 2026, filed with the SEC on March 9, 2026, and our Quarterly Report on Form 10-Q for the fiscal quarter ended April 30, 2026, filed with the SEC on June 8, 2026;

•

our Current Reports on Form 8-K filed with the SEC on November 10, 2025, December 1, 2025, December 30, 2025, January 7, 2026, February 4, 2026, April 6, 2026, May 21, 2026 (excluding Item 7.01 and Exhibit 99.1 of Item 9.01), and June 24, 2026 (excluding Item 7.01 and Exhibit 99.1 of Item 9.01);

•

our definitive proxy statement filed with the SEC on February 18, 2026; and

•

the description of our common stock set forth in our registration statement on Form 8-A, filed with the SEC on June 6, 2000, including any amendment or reports filed for the purpose of updating such description, including Exhibit 4.1 to the Company’s Annual Report on Form 10-K for the fiscal year ended October 31, 2025.

Notwithstanding the foregoing, information furnished under Items 2.02 and 7.01 of any Current Report on Form 8-K, including the related exhibits under Item 9.01, is not incorporated by reference in this prospectus supplement.

You may request a copy of any or all of these documents, which will be provided to you at no cost, by writing or telephoning us using the following contact information:

FuelCell Energy, Inc.
Attention: Corporate Secretary
3 Great Pasture Road
Danbury, Connecticut 06810
(203) 825-6000

Neither we nor the underwriters have authorized anyone to provide you with any information other than that contained in or incorporated by reference into this prospectus supplement and the accompanying prospectus and any related free writing prospectus that we authorized to be distributed to you or to which we have referred you. We and the underwriters take no responsibility for, and can provide no assurance as to the reliability of, any other information that others may give you. You should not assume that the information in this prospectus supplement or the accompanying prospectus is accurate as of any date other than the date on the front of those documents or that any document incorporated by reference is accurate as of any date other than its filing date. You should not consider this prospectus supplement or the accompanying prospectus to be an offer or solicitation relating to the securities in any jurisdiction in which such an offer or solicitation relating to the securities is not authorized. Furthermore, you should not consider this prospectus supplement or the accompanying prospectus to be an offer or solicitation relating to the securities if the person making the offer or solicitation is not qualified to do so, or if it is unlawful for you to receive such an offer or solicitation.

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PROSPECTUS

FuelCell Energy, Inc.

Common Stock
Preferred Stock
Debt Securities
Warrants
Units

This prospectus relates to securities that we may offer and sell from time to time. We may sell any combination of these securities in one or more offerings on terms to be determined at the time of sale. We will provide specific terms of the offering in supplements to this prospectus. You should read this prospectus and any prospectus supplement carefully before you invest.

Securities may be sold directly by us, through dealers or agents designated from time to time, to or through underwriters or through a combination of these methods. See “Plan of Distribution” in this prospectus for more information. We may also describe the plan of distribution for any particular offering of securities in any applicable prospectus supplement. If any agents, underwriters or dealers are involved in the sale of any securities in respect of which this prospectus is being delivered, we will disclose their names and the nature of our arrangements with them in a prospectus supplement. The net proceeds we expect to receive from any such sale will also be included in a prospectus supplement. No securities may be sold without delivery of this prospectus and the applicable prospectus supplement describing the method and terms of the offering of such securities.

In addition, selling security holders to be named in a prospectus supplement may offer and sell securities, from time to time, in such amounts as set forth in a prospectus supplement. Unless otherwise set forth in a prospectus supplement, we will not receive any proceeds from the sale of securities by any selling security holders.

Our common stock is listed on The Nasdaq Global Market under the symbol “FCEL.”

Investing in our securities involves a high degree of risk. See “Risk Factors” on page 4 of this prospectus and any other risk factors included in any accompanying prospectus supplement and in the documents incorporated by reference in this prospectus and in any accompanying prospectus supplement for a discussion of the factors you should carefully consider before deciding to purchase our securities.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the accuracy or adequacy of this prospectus. Any representation to the contrary is a criminal offense.

This prospectus may not be used to consummate sales of our securities unless, to the extent required by applicable law, it is accompanied by a prospectus supplement.

The date of this prospectus is June 8, 2026.

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	ABOUT THIS PROSPECTUS					1
	FORWARD-LOOKING STATEMENTS					1
	INCORPORATION BY REFERENCE					2
	OUR COMPANY					3
	RISK FACTORS					4
	USE OF PROCEEDS					4
	THE SECURITIES WE MAY OFFER					4
	DESCRIPTION OF CAPITAL STOCK					5
	DESCRIPTION OF DEBT SECURITIES					12
	DESCRIPTION OF WARRANTS					20
	DESCRIPTION OF UNITS					21
	SELLING SECURITY HOLDERS					22
	PLAN OF DISTRIBUTION					23
	LEGAL MATTERS					25
	EXPERTS					25
	WHERE YOU CAN FIND ADDITIONAL INFORMATION					25

Neither we nor the underwriters (if any) have authorized anyone to provide you with any information other than that contained in or incorporated by reference into this prospectus or any amendment or supplement to this prospectus or in any free writing prospectus prepared by or on behalf of us or to which we have referred you. We and the underwriters (if any) take no responsibility for, and can provide no assurance as to the reliability of, any other information that others may give you. This prospectus and any accompanying prospectus supplement do not constitute an offer to sell or the solicitation of an offer to buy any securities other than the securities described in any applicable prospectus supplement or an offer to sell or the solicitation of an offer to buy such securities in any circumstances in which such offer or solicitation is unlawful. You should assume that the information in this prospectus, any applicable prospectus supplement or any related free writing prospectus is accurate only as of the date on the front of the document and that any information we have incorporated by reference is accurate only as of the date of the document incorporated by reference, regardless of the time of delivery of this prospectus, any applicable prospectus supplement or any related free writing prospectus, or any sale of a security. Our business, financial condition, results of operations and prospects may have changed materially since those dates.

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ABOUT THIS PROSPECTUS

This prospectus is part of a registration statement that FuelCell Energy, Inc. (“FuelCell Energy,” the “Company,” “we,” “our,” “us” or similar terms) filed with the Securities and Exchange Commission (the “SEC”) using a “shelf” registration process. Under this shelf registration process, we may from time to time sell the securities or combinations of the securities described in this prospectus, and one or more of our security holders to be named in a supplement to this prospectus may from time to time sell the securities described in this prospectus, in one or more offerings. This prospectus provides you with a general description of the securities that we and our selling security holders may offer. Each time we or our selling security holders sell securities, we will, to the extent required by law, provide a prospectus supplement that will contain information about the specific terms of the offering. We may also add, update or change in any accompanying prospectus supplement any of the information contained in this prospectus. To the extent there is a conflict between the information contained in this prospectus and the prospectus supplement, you should rely on the information in the prospectus supplement, provided that if any statement in one of these documents is inconsistent with a statement in another document having a later date — for example, a document incorporated by reference in this prospectus or any prospectus supplement — the statement in the document having the later date modifies or supersedes the earlier statement. This prospectus, together with any accompanying prospectus supplement and any other document we may authorize to be delivered to you, includes all material information relating to the offering of our securities.

As permitted by the rules and regulations of the SEC, the registration statement, of which this prospectus forms a part, includes and/or incorporates by reference additional information not contained in this prospectus. You may read the registration statement and the other reports we file with the SEC at the SEC’s website as described below under the heading “Where You Can Find Additional Information.”

FORWARD-LOOKING STATEMENTS

This prospectus, any prospectus supplement, the documents incorporated by reference herein and therein and any related free writing prospectuses issued by us may contain forward-looking statements (within the meaning of the Private Securities Litigation Reform Act of 1995 (the “PSLRA”)) about our financial condition, results of operations, plans, objectives, expectations, future performance and business. Statements preceded by, followed by or that include words such as “expects,” “anticipates,” “estimates,” “goals,” “projects,” “intends,” “plans,” “believes,” “predicts,” “should,” “seeks,” “will,” “could,” “would,” “may,” “forecast,” or words or phrases of similar import are intended to identify some of the forward-looking statements and are included, along with this statement, for purposes of complying with the safe harbor provisions of the PSLRA. Forward-looking statements are neither historical facts, nor assurances of future performance. Instead, such statements are based only on our beliefs, expectations and assumptions regarding the future. As such, the realization of matters expressed in forward-looking statements involves inherent risks and uncertainties. Our actual results and future events may differ materially from those set forth in or contemplated by the forward-looking statements due to, among other factors, the risks and uncertainties described in or incorporated by reference into this prospectus, including under the heading “Risk Factors,” as well as any risks or uncertainties described in or incorporated by reference into any prospectus supplement. Any forward-looking statement contained in this prospectus, any prospectus supplement, the documents incorporated by reference herein and therein, and any related free writing prospectuses issued by us speaks only as of the date on which the statement was made, and we undertake no obligation to update publicly or revise any forward-looking statements for any reason, whether as a result of new information, future events or otherwise. New risks emerge from time to time, and we cannot predict all of the risks that may impact our business or the extent to which any particular risk, or combination of risks, may cause actual results to differ materially from those contained in any forward-looking statement.

Market data and industry forecasts and projections used in this prospectus and documents incorporated by reference herein have been obtained from independent industry sources. Forecasts, projections and other forward-looking information obtained from such sources are subject to similar qualifications and uncertainties as other forward-looking statements in this prospectus and documents incorporated by reference herein.

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INCORPORATION BY REFERENCE

The SEC allows us to “incorporate by reference” the information that we file with it, which means that we can disclose important information to you by referring you to other documents. The information incorporated by reference is an important part of this prospectus. We incorporate by reference the following documents (other than information furnished rather than filed):

•

our Annual Report on Form 10-K for the fiscal year ended October 31, 2025, filed with the SEC on December 18, 2025;

•

our Current Reports on Form 8-K, filed with the SEC on November 10, 2025, December 1, 2025, December 30, 2025, January 7, 2026, February 4, 2026, April 6, 2026, and May 21, 2026 (excluding Item 7.01 and Exhibit 99.1 of Item 9.01);

•

our definitive proxy statement filed with the SEC on February 18, 2026; and

•

the description of our common stock set forth in our registration statement on Form 8-A, filed with the SEC on June 6, 2000, including any amendments or reports filed for the purpose of updating such description, including Exhibit 4.1 to the Company’s Annual Report on Form 10-K for the fiscal year ended October 31, 2025.

All reports and other documents we file pursuant to Section 13(a), 13(c), 14 or 15(d) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”) (other than information furnished rather than filed), after the date of this prospectus and before the termination of the offering of securities pursuant to this prospectus, will also be incorporated by reference into this prospectus and deemed to be part of this prospectus from the date of the filing of such reports and documents.

We will provide without charge, upon written or oral request, a copy of any or all of the documents that are incorporated by reference into this prospectus and a copy of any or all other contracts or documents which are referred to in this prospectus. You may request a copy of any or all of these documents, which will be provided to you at no cost, by writing or telephoning us using the following contact information:

FuelCell Energy, Inc.
Attention: Corporate Secretary
3 Great Pasture Road
Danbury, Connecticut 06810
Telephone: (203) 825-6000

Our SEC filings are also available to the public on the “Investors” tab of our website at www.fuelcellenergy.com. The information on, or that can be accessed through, our website is not incorporated by reference in this prospectus, and you should not consider it to be a part of this prospectus. Our website address is included as an inactive textual reference only.

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OUR COMPANY

FuelCell Energy, Inc.

FuelCell Energy is a clean energy technology company and a stationary fuel cell manufacturer with 22 years of operating experience in this field. We were founded in 1969 to research and develop electrochemical power generation technology and are headquartered in Danbury, Connecticut. Since the early 2000s, we have been manufacturing, selling and servicing our proprietary molten carbonate fuel cell systems, which deliver large-scale, continuous clean power and advanced emissions management. Unlike traditional power generation methods that rely on combustion, our fuel cells generate electricity electrochemically through a chemical reaction rather than burning fuel, resulting in ultra-low emissions and high efficiency. Our carbonate fuel cell systems are fuel-flexible, with the ability to run on biofuels, renewable natural gas, or hydrogen-hydrocarbon blends, and provide reliable baseload power, carbon capture, and thermal energy for chilling, heating, and process steam. As global energy demand rises driven by artificial intelligence, electrification, and the need for enhanced grid resiliency, we believe solutions like ours will be vital in addressing next-generation needs, helping to strengthen the grid, reducing pollution, and supporting decarbonization goals. We have proven utility-scale projects operating at 10 MW, 20 MW, and 58.8 MW, with an average of over 10 years of continuous run time. As a company, we are motivated by our purpose of enabling a world empowered by clean energy.

Corporate Information

Our principal executive offices are located at 3 Great Pasture Road, Danbury, Connecticut 06810. Our telephone number is (203) 825-6000. We maintain a website at www.fuelcellenergy.com. The information on, or that can be accessed through, our website is not incorporated by reference in this prospectus, and you should not consider it to be a part of this prospectus. Our website address is included as an inactive textual reference only.

In this prospectus, unless otherwise stated or the context otherwise requires, references to “FuelCell Energy,” the “Company,” “we,” “us” and “our” and similar references refer to FuelCell Energy, Inc. and our consolidated subsidiaries.

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RISK FACTORS

An investment in our securities involves a high degree of risk. You should carefully consider the risk factors incorporated herein by reference to our most recent Annual Report on Form 10-K and our most recent Quarterly Report on Form 10-Q and the other information contained in this prospectus, as updated by our Quarterly Reports on Form 10-Q, Current Reports on Form 8-K, and other subsequent filings under the Exchange Act and the risk factors and other information contained or incorporated by reference in the applicable prospectus supplement before acquiring any of our securities.

USE OF PROCEEDS

Unless otherwise provided in the applicable prospectus supplement, we currently intend to use the net proceeds from our sale of securities under this prospectus for general corporate purposes, which may include future investments in businesses, products, services and/or technologies, including strategic growth and acquisition opportunities, the repayment, redemption or refinancing of indebtedness, research with respect to and the development and commercialization of our products, working capital, capital expenditures, and acquisitions and repurchases of our securities. We may set forth additional information regarding the use of proceeds from the sale of the securities we offer under this prospectus in a prospectus supplement relating to the specific offering. We have not determined the amount of net proceeds to be used specifically for the foregoing purposes. As a result, our management will have broad discretion in the allocation of the net proceeds.

We will not receive any proceeds from the sale of securities by selling security holders.

THE SECURITIES WE MAY OFFER

We may sell common stock, preferred stock, debt securities and warrants, as well as units that include any combination of the foregoing securities, in one or more offerings from time to time under this prospectus at prices and on terms to be determined at the time of any offering. This prospectus provides you with a general description of the securities we may offer. Each time we offer a type or series of securities under this prospectus, we will provide a prospectus supplement or free writing prospectus, or both, that will describe the specific amounts, prices and other important terms of the securities being offered.

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DESCRIPTION OF CAPITAL STOCK

General

Authorized and Outstanding Capital Stock

As of June 1, 2026, 67,608,173 shares of our common stock were issued and outstanding and 64,020 shares of our Series B Preferred Stock were issued and outstanding. No other shares of our preferred stock were issued and outstanding.

As of June 1, 2026, we were obligated, if and when the holders exercise their conversion rights, to issue approximately 1,261 shares of our common stock upon conversion of all of the issued and outstanding Series B Preferred Stock.

As of June 1, 2026, there were outstanding options to purchase 439 shares of our common stock under our equity incentive plan; 1,464,783 shares of our common stock were reserved for issuance upon time-based vesting of outstanding restricted stock units granted under our equity incentive plan; 652,855 shares of our common stock were reserved for potential issuance in connection with the settlement of performance-based restricted stock units granted under our equity incentive plan; 2,307,050 shares of our common stock were available for future grants of awards under our equity incentive plan; and 300,078 shares of our common stock were reserved for future issuance under our employee stock purchase plan.

As of June 1, 2026, there were 20 holders of record of our common stock.

Common Stock

Dividends. Holders of our common stock will share ratably in any dividends declared by our board of directors, subject to the preferential rights of any of our preferred stock then outstanding. (The dividend rights of the outstanding Series B Preferred Stock are described below under the heading “Series B Preferred Stock”). Dividends consisting of shares of our common stock may be paid to holders of shares of our common stock. We have never paid a cash dividend on our common stock and do not anticipate paying any cash dividends on our common stock in the foreseeable future.

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Listing on The Nasdaq Global Market. Our common stock is listed on The Nasdaq Global Market under the symbol “FCEL.”

Transfer Agent and Registrar. The transfer agent and registrar for our common stock is Equiniti Trust Company, LLC.

Preferred Stock

Our board of directors has broad discretion with respect to the creation and issuance of preferred stock without stockholder approval, subject to any applicable rights of holders of shares of any series of preferred stock outstanding from time to time. Under the terms of our Certificate of Incorporation, our board of directors is authorized, subject to any limitations prescribed by law, without stockholder approval, to issue such shares of preferred stock in one or more series. Each such series of preferred stock shall have such rights, preferences, privileges and restrictions, including voting rights, dividend rights, conversion rights, redemption privileges and liquidation preferences, as shall be determined by the board of directors.

The purpose of authorizing the board of directors to issue preferred stock and determine its rights and preferences is to eliminate delays associated with a stockholder vote on specific issuances. The issuance of preferred stock, while providing desirable flexibility in connection with possible acquisitions and other corporate purposes, could have the effect of making it more difficult for a third party to acquire, or of discouraging a third party from acquiring, a majority of our outstanding voting stock.

The holders of our common stock may be adversely affected by the rights, privileges and preferences of holders of shares of any series of preferred stock which the board of directors may designate and we may issue from time to time. The effects of issuing preferred stock could include one or more of the following:

•

decreasing the amount of earnings and assets available for distribution to holders of common stock;

•

restricting dividends on the common stock;

•

diluting the voting power of the common stock;

•

impairing the liquidation rights of the common stock; or

•

delaying, deferring or preventing changes in our control or management.

Series B Preferred Stock

Ranking. Shares of our Series B Preferred Stock rank with respect to dividend rights and rights upon our liquidation, winding up or dissolution:

•

senior to shares of our common stock;

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•

junior to our debt obligations; and

•

effectively junior to our subsidiaries’ (i) existing and future liabilities and (ii) capital stock held by others.

Liquidation. The holders of Series B Preferred Stock are entitled to receive, in the event that the Company is liquidated, dissolved or wound up, whether voluntarily or involuntarily, $1,000.00 per share plus all accumulated and unpaid dividends up to but excluding the date of such liquidation, dissolution, or winding up (the “Liquidation Preference”). Until the holders of Series B Preferred Stock receive the Liquidation Preference with respect to their shares of Series B Preferred Stock in full, no payment will be made on any junior shares, including shares of our common stock. After the Liquidation Preference is paid in full, holders of the Series B Preferred Stock will not be entitled to receive any further distribution of our assets. (For the avoidance of doubt, neither the voluntary sale of all or substantially all of our assets, nor a merger involving the Company, shall be deemed to be a voluntary or involuntary liquidation, dissolution or winding up of the Company.) As of April 30, 2026, the issued and outstanding shares of Series B Preferred Stock had an aggregate Liquidation Preference of $64.0 million.

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The adjustment of the conversion price is to prevent dilution of the interests of the holders of the Series B Preferred Stock from certain dilutive transactions with holders of our common stock.

•

Notwithstanding the foregoing, holders of shares of Series B Preferred Stock will not have the right to require us to redeem their shares if:

•

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•

in the case of a merger or consolidation constituting a fundamental change (as described in the fourth bullet above), the transaction is effected solely to change our jurisdiction of incorporation.

So long as any shares of Series B Preferred Stock remain outstanding, we will not, without the consent of the holders of at least two-thirds of the shares of Series B Preferred Stock outstanding at the time (voting separately as a class with all other series of preferred stock, if any, on parity with our Series B Preferred Stock upon which like voting rights have been conferred and are exercisable) issue or increase the authorized amount of any class or series of shares ranking senior to the outstanding shares of the Series B Preferred Stock as to dividends or upon liquidation. In addition, we will not, subject to certain conditions, amend, alter or repeal provisions of our Certificate of Incorporation, including the Series B Certificate of Designation, whether by merger, consolidation or otherwise, so as to adversely amend, alter or affect any power, preference or special right of the outstanding shares of Series B Preferred Stock or the holders thereof without the affirmative vote of not less than two-thirds of the issued and outstanding shares of Series B Preferred Stock.

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Certain Provisions of Delaware Law, Our Certificate of Incorporation, and By-laws

Anti-Takeover Provisions

•

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Exclusive Forum

Limitations of Directors’ Liability

•

for any breach of the director’s duty of loyalty to the Company or our stockholders;

•

for acts or omissions not in good faith or which involve intentional misconduct or a knowing violation of law;

•

under Section 174 of the DGCL; or

•

for any transaction from which the director derived an improper personal benefit.

The effect of these provisions is to eliminate our rights and the rights of our stockholders (through stockholders’ derivative suits on behalf of us) to recover monetary damages against a director for breach of fiduciary duty as a director (including breaches resulting from grossly negligent behavior), except in the situations described above. These provisions do not limit the liability of directors under federal securities laws and do not affect the availability of equitable remedies such as an injunction or rescission based upon a director’s breach of his duty of care.

Disclosure of Commission Position on Indemnification for Securities Act Liabilities

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DESCRIPTION OF DEBT SECURITIES

The following description, together with the additional information we include in any applicable prospectus supplement, summarizes certain general terms and provisions of the debt securities that we may offer under this prospectus. When we offer to sell a particular series of debt securities, we will describe the specific terms of the series in a supplement to this prospectus. We will also indicate in the supplement to what extent the general terms and provisions described in this prospectus apply to a particular series of debt securities.

We may issue debt securities either separately, or together with, or upon the conversion or exercise of or in exchange for, other securities described in this prospectus. Debt securities may be our senior, senior subordinated or subordinated obligations and, unless otherwise specified in a supplement to this prospectus, the debt securities will be our direct, unsecured obligations and may be issued in one or more series.

Unless otherwise specified in an accompanying prospectus supplement, the debt securities will be issued in one or more series under the indenture, dated as of April 29, 2025, among us and Wilmington Trust, National Association, as trustee. We have summarized select portions of the indenture below. The summary is not complete. The indenture has been incorporated by reference as an exhibit to the registration statement. You should read the indenture for provisions that may be important to you. Capitalized terms used in the summary have the meanings specified in the indenture. The indenture is governed by the Trust Indenture Act of 1939, as amended.

General

The terms of each series of debt securities will be established by or pursuant to a resolution of our board of directors and set forth or determined in the manner provided in a resolution of our board of directors, in an officer’s certificate or by a supplemental indenture. The particular terms of each series of debt securities will be described in a prospectus supplement relating to such series (including any pricing supplement or term sheet).

We can issue an unlimited amount of debt securities under the indenture that may be in one or more series with the same or various maturities, at par, at a premium, or at a discount. We will set forth in a prospectus supplement (including any pricing supplement or term sheet) relating to any series of debt securities being offered the aggregate principal amount and the following terms of the debt securities, if applicable:

•

the title and ranking of the debt securities (including the terms of any subordination provisions);

•

the price or prices (expressed as a percentage of the principal amount) at which we will sell the debt securities;

•

any limit upon the aggregate principal amount of the debt securities;

•

the date or dates on which the principal of the securities of the series is payable;

•

the rate or rates (which may be fixed or variable) per annum or the method used to determine the rate or rates (including any commodity, commodity index, stock exchange index or financial index) at which the debt securities will bear interest, the date or dates from which interest will accrue, the date or dates on which interest will commence and be payable and any regular record date for the interest payable on any interest payment date;

•

the place or places where principal of, and interest, if any, on the debt securities will be payable (and the method of such payment), where the securities of such series may be surrendered for registration of transfer or exchange, and where notices and demands to us in respect of the debt securities may be delivered;

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the period or periods within which, the price or prices at which and the terms and conditions upon which we may redeem the debt securities;

•

any obligation we have to redeem or purchase the debt securities pursuant to any sinking fund or analogous provisions or at the option of a holder of debt securities and the period or periods within which, the price or prices at which and the terms and conditions upon which securities of the series shall be redeemed or purchased, in whole or in part, pursuant to such obligation;

•

the dates on which and the price or prices at which we will repurchase debt securities at the option of the holders of debt securities and other detailed terms and provisions of these repurchase obligations;

•

the denominations in which the debt securities will be issued, if other than minimum denominations of $1,000 and any integral multiple thereof;

•

whether the debt securities will be issued in the form of certificated debt securities or global debt securities;

•

the portion of principal amount of the debt securities payable upon declaration of acceleration of the maturity date, if other than the principal amount;

•

the currency of denomination of the debt securities, which may be United States dollars or any foreign currency, and if such currency of denomination is a composite currency, the agency or organization, if any, responsible for overseeing such composite currency;

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the designation of the currency, currencies or currency units in which payment of principal of, premium and interest on the debt securities will be made;

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if payments of principal of, premium or interest on the debt securities will be made in one or more currencies or currency units other than that or those in which the debt securities are denominated, the manner in which the exchange rate with respect to these payments will be determined;

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the manner in which the amounts of payment of principal of, premium, if any, or interest on the debt securities will be determined, if these amounts may be determined by reference to an index based on a currency or currencies or by reference to a commodity, commodity index, stock exchange index or financial index;

•

any provisions relating to any security provided for the debt securities;

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any addition to, deletion of or change in the Events of Default described in this prospectus or in the indenture with respect to the debt securities and any change in the acceleration provisions described in this prospectus or in the indenture with respect to the debt securities;

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any addition to, deletion of or change in the covenants described in this prospectus or in the indenture with respect to the debt securities;

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any depositaries, interest rate calculation agents, exchange rate calculation agents or other agents with respect to the debt securities;

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any other terms of the debt securities, which may supplement, modify or delete any provision of the indenture as it applies to that series, including any terms that may be required under applicable law or regulations or advisable in connection with the marketing of the securities; and

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whether any of our direct or indirect subsidiaries will guarantee the debt securities of that series, including the terms of subordination, if any, of such guarantees.

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We may issue debt securities that provide for an amount less than their stated principal amount to be due and payable upon declaration of acceleration of their maturity pursuant to the terms of the indenture. We will provide you with information on the federal income tax considerations and other special considerations applicable to any of these debt securities in the applicable prospectus supplement.

If we denominate the purchase price of any of the debt securities in a foreign currency or currencies or a foreign currency unit or units, or if the principal of and any premium and interest on any series of debt securities is payable in a foreign currency or currencies or a foreign currency unit or units, we will provide you with information on the restrictions, elections, general tax considerations, specific terms and other information with respect to that issue of debt securities and such foreign currency or currencies or foreign currency unit or units in the applicable prospectus supplement.

Transfer and Exchange

Each debt security will be represented by either one or more global securities registered in the name of a clearing agency registered under the Exchange Act, which we refer to as the depositary, or a nominee of the depositary (we will refer to any debt security represented by a global debt security as a “book-entry debt security”), or a certificate issued in definitive registered form (we will refer to any debt security represented by a certificated security as a “certificated debt security”) as set forth in the applicable prospectus supplement. Except as set forth under the heading “Global Debt Securities and Book-Entry System” below, book-entry debt securities will not be issuable in certificated form.

Certificated Debt Securities

You may transfer or exchange certificated debt securities at any office we maintain for this purpose in accordance with the terms of the indenture. No service charge will be made for any transfer or exchange of certificated debt securities, but we may require payment of a sum sufficient to cover any tax or other governmental charge payable in connection with a transfer or exchange.

You may effect the transfer of certificated debt securities and the right to receive the principal of, premium and interest on certificated debt securities only by surrendering the certificate representing those certificated debt securities and either reissuance by us or the trustee of the certificate to the new holder or the issuance by us or the trustee of a new certificate to the new holder.

Global Debt Securities and Book-Entry System

Each global debt security representing book-entry debt securities will be deposited with, or on behalf of, the depositary, and registered in the name of the depositary or a nominee of the depositary.

Covenants

We will set forth in the applicable prospectus supplement any restrictive covenants applicable to any issue of debt securities.

No Protection in the Event of a Change of Control

Unless we state otherwise in the applicable prospectus supplement, the debt securities will not contain any provisions which may afford holders of the debt securities protection in the event we have a change in control or in the event of a highly leveraged transaction (whether or not such transaction results in a change in control) which could adversely affect holders of debt securities.

Conversion or Exchange Rights

For any series of debt securities that are convertible into or exchangeable for shares of our common stock, we will set forth in the applicable prospectus supplement the terms on which such series of debt securities may be convertible into or exchangeable for our common stock. We will include provisions as to settlement upon conversion or exchange and whether conversion or exchange is mandatory, at the option of the holder or at our option. We may include provisions pursuant to which the number of shares of our common stock that the holders of the series of debt securities receive would be subject to adjustment.

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Consolidation, Merger and Sale of Assets

We may not consolidate with or merge with or into, or convey, transfer or lease all or substantially all of our properties and assets to any person, which we refer to as a successor person, unless:

•

we are the surviving corporation or the successor person (if other than us) is a corporation organized and validly existing under the laws of any U.S. domestic jurisdiction and expressly assumes our obligations on the debt securities and under the indenture; and

•

immediately after giving effect to the transaction, no Default or Event of Default, shall have occurred and be continuing.

Notwithstanding the above, any of our subsidiaries may consolidate with, merge into or transfer all or part of its properties to us.

Events of Default

“Event of Default” means with respect to any series of debt securities, any of the following:

•

default in the payment of any interest upon any debt security of that series when it becomes due and payable, and continuance of such default for a period of 30 days (unless the entire amount of the payment is deposited by us with the trustee or with a paying agent prior to the expiration of the 30-day period);

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default in the payment of principal of any security of that series at its maturity;

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default in the performance or breach of any other covenant or warranty by us in the indenture (other than a covenant or warranty that has been included in the indenture solely for the benefit of a series of debt securities other than that series), which default continues uncured for a period of 60 days after we receive written notice from the trustee, or we and the trustee receive written notice from the holders of not less than 25% in principal amount of the outstanding debt securities of that series as provided in the indenture;

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certain voluntary or involuntary events of bankruptcy, insolvency or reorganization of us; and

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any other Event of Default provided with respect to debt securities of that series that is described in the applicable prospectus supplement.

No Event of Default with respect to a particular series of debt securities (except as to certain events of bankruptcy, insolvency or reorganization) necessarily constitutes an Event of Default with respect to any other series of debt securities. The occurrence of certain Events of Default or an acceleration under the indenture may constitute an event of default under certain indebtedness of ours or our subsidiaries outstanding from time to time.

We will provide the trustee written notice of any Default or Event of Default within 30 days of becoming aware of the occurrence of such Default or Event of Default, which notice will describe in reasonable detail the status of such Default or Event of Default and what action we are taking or propose to take in respect thereof.

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If an Event of Default with respect to debt securities of any series at the time outstanding occurs and is continuing, then the trustee or the holders of not less than 25% in principal amount of the outstanding debt securities of that series may, by a notice in writing to us (and to the trustee if given by the holders), declare to be due and payable immediately the principal of (or, if the debt securities of that series are discount securities, that portion of the principal amount as may be specified in the terms of that series) and accrued and unpaid interest, if any, on all debt securities of that series. In the case of an Event of Default resulting from certain events of bankruptcy, insolvency or reorganization, the principal (or such specified amount) of and accrued and unpaid interest, if any, on all outstanding debt securities will become and be immediately due and payable without any declaration or other act on the part of the trustee or any holder of outstanding debt securities. At any time after a declaration of acceleration with respect to debt securities of any series has been made, but before a judgment or decree for payment of the money due has been obtained by the trustee, the holders of a majority in principal amount of the outstanding debt securities of that series may rescind and annul the acceleration if all Events of Default, other than the non-payment of accelerated principal and interest, if any, with respect to debt securities of that series, have been cured or waived as provided in the indenture. We refer you to the prospectus supplement relating to any series of debt securities that are discount securities for the particular provisions relating to acceleration of a portion of the principal amount of such discount securities upon the occurrence of an Event of Default.

The indenture provides that the trustee may refuse to perform any duty or exercise any of its rights or powers under the indenture unless the trustee receives indemnity satisfactory to it against any cost, liability or expense which might be incurred by it in performing such duty or exercising such right or power. Subject to certain rights of the trustee, the holders of a majority in principal amount of the outstanding debt securities of any series will have the right to direct the time, method and place of conducting any proceeding for any remedy available to the trustee or exercising any trust or power conferred on the trustee with respect to the debt securities of that series.

No holder of any debt security of any series will have any right to institute any proceeding, judicial or otherwise, with respect to the indenture or for the appointment of a receiver or trustee, or for any remedy under the indenture, unless:

•

that holder has previously given to the trustee written notice of a continuing Event of Default with respect to debt securities of that series; and

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the holders of not less than 25% in principal amount of the outstanding debt securities of that series have made written request, and offered indemnity or security satisfactory to the trustee, to the trustee to institute the proceeding as trustee, and the trustee has not received from the holders of not less than a majority in principal amount of the outstanding debt securities of that series a direction inconsistent with that request and has failed to institute the proceeding within 60 days.

Notwithstanding any other provision in the indenture, the holder of any debt security will have an absolute and unconditional right to receive payment of the principal of, premium and any interest on that debt security on or after the due dates expressed in that debt security and to institute suit for the enforcement of payment.

The indenture requires us, within 120 days after the end of our fiscal year, to furnish to the trustee a statement as to compliance with the indenture. If a Default or Event of Default occurs and is continuing with respect to the securities of any series and if it is actually known to a responsible officer of the trustee, the trustee shall send to each securityholder of the securities of that series notice of a Default or Event of Default within 90 days after it occurs or, if later, after a responsible officer of the trustee has knowledge of such Default or Event of Default. The indenture provides that the trustee may withhold notice to the holders of debt securities of any series of any Default or Event of Default (except in payment on any debt securities of that series) with respect to debt securities of that series if the trustee determines in good faith that withholding notice is in the interest of the holders of those debt securities.

Modification and Waiver

We and the trustee may modify, amend or supplement the indenture or the debt securities of any series without the consent of any holder of any debt security:

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to cure any ambiguity, defect or inconsistency;

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to comply with covenants in the indenture described above under the heading “Consolidation, Merger and Sale of Assets”;

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to provide for uncertificated securities in addition to or in place of certificated securities;

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to add guarantees with respect to debt securities of any series or secure debt securities of any series;

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to surrender any of our rights or powers under the indenture;

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to add covenants or events of default for the benefit of the holders of debt securities of any series;

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to comply with the applicable procedures of the applicable depositary;

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to make any change that does not adversely affect the rights of any holder of debt securities;

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to provide for the issuance of and establish the form and terms and conditions of debt securities of any series as permitted by the indenture;

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to effect the appointment of a successor trustee or a separate trustee with respect to the debt securities of any series and to add to or change any of the provisions of the indenture to provide for or facilitate administration by more than one trustee; or

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to comply with requirements of the SEC in order to effect or maintain the qualification of the indenture under the Trust Indenture Act.

We may also modify and amend the indenture with the consent of the holders of at least a majority in principal amount of the outstanding debt securities of each series affected by the modifications or amendments. We may not make any modification or amendment without the consent of the holders of each affected debt security then outstanding if that amendment will:

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reduce the amount of debt securities whose holders must consent to an amendment, supplement or waiver;

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reduce the rate of or extend the time for payment of interest (including default interest) on any debt security;

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reduce the principal of or premium on or change the fixed maturity of any debt security or reduce the amount of, or postpone the date fixed for, the payment of any sinking fund or analogous obligation with respect to any series of debt securities;

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reduce the principal amount of discount securities payable upon acceleration of maturity;

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waive a default in the payment of the principal of, premium or interest on any debt security (except a rescission of acceleration of the debt securities of any series by the holders of at least a majority in aggregate principal amount of the then outstanding debt securities of that series and a waiver of the payment default that resulted from such acceleration);

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make the principal of or premium or interest on any debt security payable in currency other than that stated in the debt security;

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make any change to certain provisions of the indenture relating to, among other things, the right of holders of debt securities to receive payment of the principal of, premium and interest on those debt securities and to institute suit for the enforcement of any such payment and to waivers or amendments; or

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waive a redemption payment with respect to any debt security.

Except for certain specified provisions, the holders of at least a majority in principal amount of the outstanding debt securities of any series may on behalf of the holders of all debt securities of that series waive our compliance with provisions of the indenture. The holders of a majority in principal amount of the outstanding debt securities of any series may on behalf of the holders of all the debt securities of such series waive any past default under the indenture with respect to that series and its consequences, except a default in the payment of the principal of, premium or any interest on any debt security of that series; provided, however, that the holders of a majority in principal amount of the outstanding debt securities of any series may rescind an acceleration and its consequences, including any related payment default that resulted from the acceleration.

Defeasance of Debt Securities and Certain Covenants in Certain Circumstances

Legal Defeasance

The indenture provides that, unless otherwise provided by the terms of the applicable series of debt securities, we may be discharged from any and all obligations in respect of the debt securities of any series (subject to certain exceptions). We will be so discharged upon the irrevocable deposit with the trustee, in trust, of money and/or U.S. government obligations or, in the case of debt securities denominated in a single currency other than U.S. dollars, government obligations of the government that issued or caused to be issued such currency, that, through the payment of interest and principal in accordance with their terms, will provide money or U.S. government obligations in an amount sufficient in the opinion of a nationally recognized firm of independent public accountants or investment bank to pay and discharge each installment of principal, premium and interest on and any mandatory sinking fund payments in respect of the debt securities of that series on the stated maturity of those payments in accordance with the terms of the indenture and those debt securities.

This discharge may occur only if, among other things, we have delivered to the trustee an opinion of counsel stating that we have received from, or there has been published by, the United States Internal Revenue Service a ruling or, since the date of execution of the indenture, there has been a change in the applicable United States federal income tax law, in either case to the effect that, and based thereon such opinion shall confirm that, the holders of the debt securities of that series will not recognize income, gain or loss for United States federal income tax purposes as a result of the deposit, defeasance and discharge and will be subject to United States federal income tax on the same amounts and in the same manner and at the same times as would have been the case if the deposit, defeasance and discharge had not occurred.

Defeasance of Certain Covenants

The indenture provides that, unless otherwise provided by the terms of the applicable series of debt securities, upon compliance with certain conditions:

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we may omit to comply with the covenant described under the heading “Consolidation, Merger and Sale of Assets” and certain other covenants set forth in the indenture, as well as any additional covenants which may be set forth in the applicable prospectus supplement; and

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any omission to comply with those covenants will not constitute a Default or an Event of Default with respect to the debt securities of that series.

We refer to this as covenant defeasance. The conditions include:

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depositing with the trustee money and/or U.S. government obligations or, in the case of debt securities denominated in a single currency other than U.S. dollars, government obligations of the government that issued or caused to be issued such currency, that, through the payment of interest and principal in accordance with their terms, will provide money in an amount sufficient in the opinion of a nationally recognized firm of independent public accountants or investment bank to pay and discharge each installment of principal of, premium and interest on and any mandatory sinking fund payments in respect of the debt securities of that series on the stated maturity of those payments in accordance with the terms of the indenture and those debt securities;

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such deposit will not result in a breach or violation of, or constitute a default under the indenture or any other agreement to which we are a party;

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no Default or Event of Default with respect to the applicable series of debt securities shall have occurred or is continuing on the date of such deposit; and

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delivering to the trustee an opinion of counsel to the effect that we have received from, or there has been published by, the United States Internal Revenue Service a ruling or, since the date of execution of the indenture, there has been a change in the applicable United States federal income tax law, in either case to the effect that, and based thereon such opinion shall confirm that, the holders of the debt securities of that series will not recognize income, gain or loss for United States federal income tax purposes as a result of the deposit and related covenant defeasance and will be subject to United States federal income tax on the same amounts and in the same manner and at the same times as would have been the case if the deposit and related covenant defeasance had not occurred.

No Personal Liability of Directors, Officers, Employees or Stockholders

None of our past, present or future directors, officers, employees or stockholders, as such, will have any liability for any of our obligations under the debt securities or the indenture or for any claim based on, or in respect or by reason of, such obligations or their creation. By accepting a debt security, each holder waives and releases all such liability. This waiver and release is part of the consideration for the issue of the debt securities. However, this waiver and release may not be effective to waive liabilities under U.S. federal securities laws, and it is the view of the SEC that such a waiver is against public policy.

Governing Law

The indenture and the debt securities, including any claim or controversy arising out of or relating to the indenture or the securities, will be governed by the laws of the State of New York.

The indenture will provide that we, the trustee and the holders of the debt securities (by their acceptance of the debt securities) irrevocably waive, to the fullest extent permitted by applicable law, any and all right to trial by jury in any legal proceeding arising out of or relating to the indenture, the debt securities or the transactions contemplated thereby. The indenture will provide that any legal suit, action or proceeding arising out of or based upon the indenture or the transactions contemplated thereby may be instituted in the federal courts of the United States of America located in the City of New York or the courts of the State of New York in each case located in the City of New York, and we, the trustee and the holder of the debt securities (by their acceptance of the debt securities) irrevocably submit to the non-exclusive jurisdiction of such courts in any such suit, action or proceeding. The indenture will further provide that service of any process, summons, notice or document by mail (to the extent allowed under any applicable statute or rule of court) to such party’s address set forth in the indenture will be effective service of process for any suit, action or other proceeding brought in any such court. The indenture will further provide that we, the trustee and the holders of the debt securities (by their acceptance of the debt securities) irrevocably and unconditionally waive any objection to the laying of venue of any suit, action or other proceeding in the courts specified above and irrevocably and unconditionally waive and agree not to plead or claim any such suit, action or other proceeding has been brought in an inconvenient forum.

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DESCRIPTION OF WARRANTS

We may issue warrants in the future for the purchase of debt securities, common stock, preferred stock, units or other securities. Warrants may be issued independently or together with debt securities, common stock, preferred stock or units offered by any prospectus supplement and/or other offering material and may be attached to or separate from any such offered securities. Each series of warrants will be issued under a separate warrant agreement to be entered into between us and a bank or trust company, as warrant agent, provided that we may also act as warrant agent and enter into warrant agreements directly with the purchasers of securities offered pursuant to this prospectus. In each case, the terms of the warrants will be set forth in a prospectus supplement and/or other offering material relating to the particular issue of warrants. The warrant agent, if any, will act solely as our agent in connection with the warrants and will not assume any obligation or relationship of agency or trust for or with any holders of warrants or beneficial owners of warrants.

The following summary of certain provisions of the warrants we may issue in the future does not purport to be complete and is subject to, and is qualified in its entirety by reference to, all provisions of the warrant agreements.

Reference is made to the prospectus supplement and/or other offering material relating to the particular issue of warrants for the terms of and information relating to such warrants, including, where applicable:

•

the designation, aggregate principal amount, currencies, denominations and terms of the series of debt securities purchasable upon exercise of warrants to purchase debt securities and the price at which such debt securities may be purchased upon such exercise;

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the number of shares of common stock or preferred stock purchasable upon the exercise of warrants and the price at which such number of shares of common stock or preferred stock may be purchased upon such exercise;

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the designation and number of units of other securities purchasable upon the exercise of warrants to purchase other securities and the price at which such number of units of such other securities may be purchased upon such exercise;

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the date on which the right to exercise such warrants shall commence and the date on which such right shall expire;

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certain material U.S. federal income tax considerations applicable to such warrants;

•

the amount of warrants outstanding as of the most recent practicable date; and

•

any other terms of such warrants.

Warrants will be issued in registered form only. The exercise price for warrants will be subject to adjustment as set forth in the applicable prospectus supplement and/or other offering material.

Each warrant will entitle the holder thereof to purchase such principal amount of debt securities or such number of shares of common stock, preferred stock, units or other securities at such exercise price as shall in each case be set forth in, or calculable from, the prospectus supplement and/or other offering material relating to the warrants, which exercise price may be subject to adjustment upon the occurrence of certain events as set forth in such prospectus supplement and/or other offering material. After the close of business on the expiration date, or such later date to which such expiration date may be extended by us, unexercised warrants will become void. The place or places where, and the manner in which, warrants may be exercised shall be specified in the prospectus supplement and/or other offering material relating to such warrants.

Prior to the exercise of any warrants, holders of such warrants will not have any of the rights of holders of the underlying securities, as the case may be, purchasable upon such exercise, including the right to receive payments of dividends, if any, or to exercise any applicable right to vote.

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DESCRIPTION OF UNITS

As specified in the applicable prospectus supplement, we may issue units consisting of one or more shares of common stock, shares of preferred stock, debt securities and warrants. The applicable prospectus supplement will specify the following terms of the units:

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the terms of the underlying securities comprising the units, including whether and under what circumstances the underlying securities may be traded separately;

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a description of the terms of any unit agreement governing the units (if any);

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if appropriate, a discussion of certain material U.S. federal income tax considerations applicable to such units; and

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a description of the provisions for the payment, settlement, transfer or exchange of the units.

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SELLING SECURITY HOLDERS

Information about selling security holders, where applicable, will be set forth in a prospectus supplement, in a post-effective amendment, or in filings we make with the SEC under the Exchange Act which are incorporated by reference.

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PLAN OF DISTRIBUTION

The securities may be sold:

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to or through underwriting syndicates represented by managing underwriters;

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through one or more underwriters without a syndicate for them to offer and sell to the public;

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through dealers or agents;

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in “at the market offerings” to or through a market maker or into an existing trading market, or a securities exchange or otherwise;

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to investors directly in negotiated sales or in competitively bid transactions; or

•

as otherwise described in a prospectus supplement, including through a combination of any of these methods of sale.

We will describe the plan of distribution for any particular offering of securities in the applicable prospectus supplement, in accordance with applicable law. The prospectus supplement will set forth the terms of the offering of securities, including the following:

•

the name or names of any underwriters, dealers or agents;

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the purchase price, the proceeds from that sale and the expected use of such proceeds;

•

any options under which underwriters may purchase additional securities from us or from a selling security holder participating in such offering;

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any underwriting discounts and other items constituting underwriters’ compensation;

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any initial public offering price and any discounts or concessions allowed or reallowed or paid to underwriters, dealers or agents; and

•

any securities exchanges on which the securities may be listed.

The securities may be distributed from time to time in one or more transactions, at negotiated prices, at a fixed price or fixed prices (that may be subject to change), at market prices prevailing at the time of sale, at various prices determined at the time of sale or at prices related to prevailing market prices.

Underwriters

If underwriters are used in the sale, we will execute an underwriting agreement with the underwriters relating to the securities that we will offer. Unless otherwise set forth in the prospectus supplement, the obligations of the underwriters to purchase these securities will be subject to conditions. The underwriters will be obligated to purchase all of the offered securities if any are purchased.

The securities subject to the underwriting agreement will be acquired by the underwriters for their own account and may be resold by them from time to time in one or more transactions, including negotiated transactions, at a fixed public offering price or at varying prices determined at the time of sale. Underwriters may be deemed to have received compensation from us in the form of underwriting discounts or commissions and may also receive commissions from the purchasers of these securities for whom they may act as agent. Underwriters may sell these securities to or through dealers. These dealers may receive compensation in the form of discounts, concessions or commissions from the underwriters and/or commissions from the purchasers for whom they may act as agent. Any initial public offering price and any discounts or concessions allowed or reallowed or paid to dealers may be changed from time to time.

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We also may sell the securities in connection with a remarketing upon their purchase, in connection with a redemption or repayment, by a remarketing firm acting as principal for its own account or as our agent. Remarketing firms may be deemed to be underwriters in connection with the securities that they remarket.

We may authorize underwriters to solicit offers by institutions to purchase the securities subject to the underwriting agreement from us, at the public offering price stated in the prospectus supplement under delayed delivery contracts providing for payment and delivery on a specified date in the future. If we sell securities under these delayed delivery contracts, the prospectus supplement will state that as well as the conditions to which these delayed delivery contracts will be subject and the commissions payable for that solicitation.

Any selling security holders, dealers and agents participating in the distribution of securities may be deemed to be underwriters, and compensation received by them on resale of the securities may be deemed to be underwriting discounts. Additionally, because selling security holders may be deemed to be “underwriters” within the meaning of Section 2(11) of the Securities Act, selling security holders may be subject to the prospectus delivery requirements of the Securities Act.

Agents

We may also sell any of the securities through agents designated by us from time to time. We will name any agent involved in the offer or sale of these securities and will list commissions payable by us to any such agents in the prospectus supplement. These agents will be acting on a best efforts basis to solicit purchases for the period of their appointment, unless we state otherwise in the prospectus supplement.

Direct Sales

We may sell any of the securities directly to purchasers. In this case, we will not engage underwriters or agents in the offer and sale of these securities.

Indemnification

We may indemnify underwriters, dealers or agents who participate in the distribution of securities against certain liabilities, including liabilities under the Securities Act and agree to contribute to payments which these underwriters, dealers or agents may be required to make. The terms and conditions of any indemnification or contribution will be described in the applicable prospectus supplement.

Secondary Sales

Our securities may be sold from time to time by selling security holders, through public or private transactions at prevailing market prices or at privately negotiated prices, as described in the applicable prospectus supplement.

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LEGAL MATTERS

The validity of the securities covered by this prospectus will be passed upon for us by Foley & Lardner LLP. The validity of the securities covered by this prospectus will be passed upon for any underwriters or agents by counsel named in the applicable prospectus supplement. The opinions of Foley & Lardner LLP and counsel for any underwriters or agents may be conditioned upon and may be subject to assumptions regarding future action required to be taken by us and any underwriters, dealers or agents in connection with the issuance of any securities. The opinions of Foley & Lardner LLP and counsel for any underwriters or agents may be subject to other conditions and assumptions, as indicated in the applicable prospectus supplement.

EXPERTS

WHERE YOU CAN FIND ADDITIONAL INFORMATION

We file annual, quarterly and current reports, proxy statements and other information with the SEC. Our SEC filings are available to the public at the SEC’s website at http://www.sec.gov.

This prospectus is part of a registration statement that we have filed with the SEC relating to the securities to be offered. This prospectus does not contain all of the information we have included in the registration statement and the accompanying exhibits and schedules in accordance with the rules and regulations of the SEC, and we refer you to the omitted information. The statements this prospectus makes pertaining to the content of any contract, agreement or other document that is an exhibit to the registration statement necessarily are summaries of their material provisions and do not describe all provisions, exceptions and qualifications contained in those contracts, agreements or documents. You should read those contracts, agreements or documents for information that may be important to you. The registration statement, exhibits and schedules are available at the SEC’s website at http://www.sec.gov.

Neither we nor the underwriters (if any) have authorized anyone to provide you with any information other than that contained in or incorporated by reference into this prospectus or any amendment or supplement to this prospectus or in any free writing prospectus prepared by or on behalf of us or to which we have referred you. We and the underwriters (if any) take no responsibility for, and can provide no assurance as to the reliability of, any other information that others may give you. You should not assume that the information in this prospectus or any prospectus supplement is accurate as of any date other than the date on the front of those documents or that any document incorporated by reference is accurate as of any date other than its filing date. You should not consider this prospectus to be an offer or solicitation relating to the securities in any jurisdiction in which such an offer or solicitation relating to the securities is not authorized. Furthermore, you should not consider this prospectus to be an offer or solicitation relating to the securities if the person making the offer or solicitation is not qualified to do so, or if it is unlawful for you to receive such an offer or solicitation.

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10,714,286 Shares

FuelCell Energy, Inc.

Common Stock

P R O S P E C T U S S U P P L E M E N T

July 7, 2026

Joint Lead Book-Running Managers

Citigroup

Barclays

Book-Running Managers

Oppenheimer & Co.

RBC Capital Markets

Goldman Sachs & Co. LLC

Co-Managers

Canaccord Genuity

B. Riley Securities

BMO Capital Markets

Siebert Williams Shank

Tuohy Brothers