燃料电池能源美股招股说明书(2025-12-30版)

Up to $200,000,000 FuelCell Energy,Inc. Common Stock This prospectus supplement (“Prospectus Supplement No.1”) amends and supplements the information in the prospectussupplement dated March10, 2025 (the “Prospectus Supplement”) relating to the offer and sale of shares of our common stock, $0.0001 par value per share (“common stock”), pursuant to an Open Market Sale AgreementSM(as amended, the “Sales Agreement”)with Jefferies LLC, B. Riley Securities,Inc., Barclays Capital Inc., BMO Capital Markets Corp., BofA Securities,Inc., CanaccordGenuity LLC, Citigroup Global Markets Inc. and Loop Capital Markets LLC, as sales agents (the “Sales Agents”). On December30, 2025, we entered into an amendment to the Sales Agreement to increase the amount of shares of common stockthat may be sold under the Sales Agreement to $200,000,000 (exclusive of any sales made prior to the date of the amendment).Accordingly, as of the date of this Prospectus Supplement No.1, shares of our common stock having an aggregate offering price of upto $200,000,000 are available for offer and sale pursuant to the Sales Agreement, the Prospectus Supplement and this Prospectus This Prospectus Supplement No.1 is not complete without and may only be delivered or utilized in connection with theProspectus Supplement, the accompanying base prospectus, and any future amendments or supplements thereto. Our common stock is listed on The Nasdaq Global Market under the symbol “FCEL.” On December29, 2025, the last reportedsale price for our common stock was $8.16 per share. Investing in our common stock involves a high degree of risk. You should read the Prospectus Supplement and theaccompanying prospectus carefully before you make your investment decision. See “Risk Factors” beginning on pageS-6ofthe Prospectus Supplement, as well as the documents we file with the Securities and Exchange Commission that are Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved ofthese securities or determined if the Prospectus Supplement or the accompanying prospectus is truthful or complete. Anyrepresentation to the contrary is a criminal offense. BofASecurities