Exhibit 5.1

ATTORNEYS AT LAW   111 HUNTINGTON AVENUE BOSTON, MASSACHUSETTS 02199 617.342.4000 TEL 617.342.4001 FAX WWW.FOLEY.COM   CLIENT/MATTER NUMBER 106876-0174

April 10, 2024

FuelCell Energy, Inc.

3 Great Pasture Road

Danbury, Connecticut 06810

Ladies and Gentlemen:

We have acted as counsel to FuelCell Energy, Inc., a Delaware corporation (the “Company”), in connection with the Company’s issuance and sale, through Jefferies LLC, B. Riley Securities, Inc., Barclays Capital Inc., BMO Capital Markets Corp., BofA Securities, Inc., Canaccord Genuity LLC, Citigroup Global Markets Inc., J.P. Morgan Securities LLC and Loop Capital Markets LLC, as sales agents and/or principals (the “Sales Agents”), of shares of the Company’s common stock, par value $0.0001 per share, having an aggregate offering price of up to $300,000,000 (the “Shares”), from time to time and at various prices in an “at the market” offering pursuant to (i) that certain Open Market Sale Agreement, dated July 12, 2022 and amended as of April 10, 2024 (as so amended, the “Amended Sales Agreement”), by and among the Company and the Sales Agents, and (ii) the Company’s Registration Statement on Form S-3 (File No. 333-274971), filed with the Securities and Exchange Commission (the “Commission”) and effective on October 13, 2023 (the “Registration Statement”), the base prospectus filed as part of the Registration Statement (the “Base Prospectus”), and the final prospectus supplement dated April 10, 2024 (together with the Base Prospectus, the “Prospectus”).

In connection with our representation, we have examined: (i) the Amended Sales Agreement, (ii) the Registration Statement and the Prospectus, (iii) the Certificate of Incorporation of the Company, as amended, (iv) the Second Amended and Restated By-laws of the Company, as amended and restated, and (v) the proceedings and actions taken by the Board of Directors of the Company to authorize and approve the transactions contemplated by the Amended Sales Agreement, including the sale and issuance of the Shares (the “Board Resolutions”). We have also considered such matters of law and of fact, including the examination of originals or copies, certified or otherwise identified to our satisfaction, of such records and documents of the Company, certificates of officers, directors and representatives of the Company, certificates of public officials, and such other documents as we have deemed appropriate as a basis for the opinions set forth below. In our examination of the above-referenced documents, we have assumed the genuineness of all signatures, the authenticity of all documents, records, certificates, and instruments submitted to us as originals and the conformity with the originals of all documents, records, certificates, and instruments submitted to us as copies. In addition, in expressing the opinion set forth below, we have assumed that the Company will issue the Shares in accordance with the Board Resolutions.

April 10, 2024

Page 2

Our opinions expressed herein are limited to the General Corporation Law of the State of Delaware, and we express no opinion as to the laws of any other jurisdiction.

Based upon, subject to and limited by the foregoing, we are of the opinion that, upon the issuance of the Shares pursuant to the terms of the Amended Sales Agreement and the receipt by the Company of the consideration for the Shares pursuant to the terms of the Amended Sales Agreement, the Shares will be validly issued, fully paid, and nonassessable.

This opinion is issued as of the date hereof, and we assume no obligation to supplement this opinion if any applicable law changes after the date hereof or if we become aware of any fact that might change the opinion expressed herein after the date hereof. This opinion is limited to the matters set forth herein, and no other opinion should be inferred beyond the matters expressly stated.

We consent to the filing of this opinion in accordance with the requirements of Item 601(b)(5) of Regulation S-K under the Securities Act of 1933, as amended (the “Securities Act”), as Exhibit 5.1 to the Company’s Current Report on Form 8-K filed with the Commission on the date hereof, to the incorporation by reference of this opinion into the Registration Statement and the Prospectus and to the references to our firm therein. In giving our consent, we do not admit that we are “experts” within the meaning of Section 11 of the Securities Act or within the category of persons whose consent is required by Section 7 of the Securities Act.

Exhibit 10.1 

AMENDMENT NO. 1 TO THE OPEN MARKET SALE AGREEMENTSM

April 10, 2024

JEFFERIES LLC

520 Madison Avenue

New York, New York 10022

B. RILEY SECURITIES, INC.

299 Park Avenue, 7th Floor

New York, New York 10171

BARCLAYS CAPITAL INC.

745 7th Avenue

New York, New York 10019

BMO CAPITAL MARKETS CORP.

151 West 42nd Street, 32nd Floor

New York, New York 10036

BOFA SECURITIES, INC.

One Bryant Park

New York, New York 10036

CANACCORD GENUITY LLC

One Post Office Square

30th Floor, Suite 3000

Boston, Massachusetts 02109

CITIGROUP GLOBAL MARKETS INC.

388 Greenwich Street

New York, New York 10013

J.P. MORGAN SECURITIES LLC

383 Madison Avenue

New York, New York 10179

LOOP CAPITAL MARKETS LLC

425 South Financial Place, Suite 2700

Chicago, Illinois 60605 USA

Ladies and Gentlemen:

This Amendment No. 1 to the Open Market Sale AgreementSM (this “Amendment”) is entered into as of the date first written above by FuelCell Energy, Inc., a Delaware corporation (the “Company”), and Jefferies LLC, B. Riley Securities, Inc., Barclays Capital Inc., BMO Capital Markets Corp., BofA Securities, Inc., Canaccord Genuity LLC, Citigroup Global Markets Inc., J.P. Morgan Securities LLC and Loop Capital Markets LLC (each individually, an “Agent” and together, the “Agents”), who are parties to that certain Open Market Sale AgreementSM, dated July 12, 2022 (the “Original Agreement”) relating to the offering of up to 95,000,000 of the Company’s Common Shares (as defined below).

All capitalized terms not defined herein shall have the meanings ascribed to them in the Original Agreement.

The parties, intending to be legally bound, hereby amend the Original Agreement, as of the date hereof, as follows:

1.     The first paragraph of the Original Agreement is hereby deleted in its entirety and replaced with the following:

“FuelCell Energy, Inc., a Delaware corporation (the “Company”), proposes, subject to the terms and conditions stated herein, to issue and sell from time to time through Jefferies LLC, B. Riley Securities, Inc., Barclays Capital Inc., BMO Capital Markets Corp., BofA Securities, Inc., Canaccord Genuity LLC, Citigroup Global Markets Inc., J.P. Morgan Securities LLC and Loop Capital Markets LLC, as sales agents and/or principals (each individually, an “Agent” and collectively, the “Agents”), shares of the Company’s common stock, par value $0.0001 per share (the “Common Shares”), having an aggregate offering price of up to $300,000,000 (exclusive of any sales of Common Shares prior to the date of this Amendment) on the terms set forth in this agreement (this “Agreement”).”

2.     The definition of “Maximum Program Amount” is hereby deleted in its entirety and replaced with the following:

“‘Maximum Program Amount’ means Common Shares with an aggregate Sales Price of the lesser of (a) the number or dollar amount of Common Shares registered under the effective Registration Statement (as defined below) pursuant to which the offering is being made, (b) the number of authorized but unissued Common Shares (less Common Shares issuable upon exercise, conversion or exchange of any outstanding securities of the Company or otherwise reserved from the Company’s authorized capital stock), (c) the number or dollar amount of Common Shares permitted to be sold under Form S-3ASR (including General Instruction I.B.6 thereof, if applicable), or (d) the number or dollar amount of Common Shares for which the Company has filed a Prospectus (as defined below).”

3.     The first sentence of Section 2(a) of the Original Agreement is hereby deleted and replaced with the following:

“Registration Statement. The Company has prepared and filed with the Commission a shelf registration statement on Form S-3ASR (File No. 333-274971) that contains a base prospectus (the “Base Prospectus”).”

“Sale of Securities. On the basis of the representations, warranties and agreements herein contained, but subject to the terms and conditions herein set forth, the Company and the Agents agree that the Company may from time to time seek to sell Shares through the Agents, acting as sales agents, or directly to the Agents, acting as principals, as follows, with an aggregate Sales Price of up to the Maximum Program Amount, based on and in accordance with such Issuance Notices as the Company may deliver, during the Agency Period.”

“Issuance Notice. Upon the terms and subject to the conditions set forth herein, on any Trading Day during the Agency Period on which the conditions set forth in Section 5(a) and Section 5(b) shall have been satisfied, the Company may exercise its right to request an issuance of Shares by delivering to an Agent (the “Designated Agent”) an Issuance Notice; provided, however, that (A) in no event may the Company deliver an Issuance Notice to the extent that the sum of (x) the aggregate requested Issuance Amount, plus (y) the aggregate Sales Price of all Shares issued under all previous Issuance Notices effected pursuant to this Agreement, would exceed the Maximum Program Amount; and (B) prior to delivery of any Issuance Notice, the period set forth for any previous Issuance Notice shall have expired or been terminated. An Issuance Notice shall be considered delivered on the Trading Day on which it is received by e-mail to the persons designated by the Designated Agent in Schedule A hereto and confirmed by the Company by telephone (including a voicemail message to the persons so identified), with the understanding that, with adequate prior written notice, the Designated Agent may modify the list of such persons from time to time.”

6.     Without limiting any other agreement, including Section 3(d) of the Original Agreement, the Company agrees to pay all costs, fees and expenses incurred in connection with entering into this Amendment and the performance of the Company’s obligations under the Original Agreement as amended by this Amendment and the transactions contemplated hereby and thereby, including without limitation, (i) all expenses incident to the issuance and delivery of the Shares (including all printing and engraving costs); (ii) all fees and expenses of the registrar and transfer agent of the Shares; (iii) all necessary issue, transfer and other stamp taxes in connection with the issuance and sale of the Shares; (iv) all fees and expenses of the Company’s counsel, independent public or certified public accountants and other advisors; (v) all costs and expenses incurred in connection with the preparation, printing, filing, shipping and distribution of the Registration Statement (including financial statements, exhibits, schedules, consents and certificates of experts), the Prospectus, any Free Writing Prospectus prepared by or on behalf of, used by, or referred to by the Company, and all amendments and supplements thereto, and this Amendment; (vi) all filing fees, attorneys’ fees and expenses incurred by the Company or the Agents in connection with qualifying or registering (or obtaining exemptions from the qualification or registration of) all or any part of the Shares authorized by this Amendment for offer and sale under the state securities or blue sky laws or the provincial securities laws of Canada, and, if requested by the Agents, preparing and printing a “Blue Sky Survey” or memorandum and a “Canadian wrapper” and any supplements thereto, advising the Agents of such qualifications, registrations, determinations and exemptions; (vii) the reasonable fees and disbursements of the Agents’ counsel, including the reasonable and documented fees and expenses of counsel for the Agents in connection with, FINRA review, if any, and approval of the Agents’ participation in the offering and distribution of the Shares authorized by this Amendment, provided, however, that the reasonable and documented fees and expenses of counsel for the Agents in connection with FINRA review and approval of the Agents’ participation in the offering and distribution of the Shares authorized by this Amendment and the Company’s filings with FINRA shall not exceed $15,000 (excluding the FINRA filing fees previously paid and referred to in Section 3(d)(viii) of the Original Agreement, and including the filing fees referred to in clause (viii) below); (viii) the filing fees incident to FINRA review, if any; (ix) the costs and expenses of the Company relating to investor presentations on any “road show” undertaken in connection with the marketing of the offering of the Shares, including, without limitation, expenses associated with the preparation or dissemination of any electronic road show, expenses associated with the production of road show slides and graphics, fees and expenses of any consultants engaged in connection with the road show presentations with the prior approval of the Company, travel and lodging expenses of the representatives, employees and officers of the Company and of the Agents and any such consultants with the prior approval of the Company, and the cost of any aircraft chartered in connection with the road show with the prior approval of the Company; (x) the fees and expenses associated with listing the Shares on the Principal Market; and (xi) the fees and expenses incurred in connection with the execution of this Amendment, which such amount is due upon the execution of this Amendment. The fees and disbursements of Agents’ counsel that may be payable by the Company pursuant to subsections (i)-(xi) above shall not exceed $75,000. For the avoidance of doubt, the $75,000 of fees and disbursements of Agents’ counsel that may become payable by the Company hereunder shall be in addition to the fees and disbursements of Agents’ counsel that are payable by the Company under Section 3(d) of the Original Agreement in connection with each Triggering Event Date on which the Company is required to provide a certificate pursuant to Section 5(o) of the Original Agreement.

7.     All references in the Original Agreement to the “Agreement” shall mean the Original Agreement as amended by this Amendment; provided, however, that all references to the “date of this Agreement” or the “date hereof” in the Original Agreement shall continue to refer to the date of the Original Agreement, unless otherwise amended herein and except with respect to the first paragraph of Section 2, Section 2(b) “Compliance with Registration Requirements,” Section 2(s) “Non-Contravention of Existing Instruments; No Further Authorizations or Approvals Required,” Section 4(f) “Free Writing Prospectuses,” Section 4(p) “Company Counsel Legal Opinion,” Section 4(r) “Comfort Letter,” and Section 8(a) “Press Releases and Disclosure,” where references to the “date of this Agreement” or the “date hereof” in the Original Agreement shall refer to each of the date of the Original Agreement and the date of this Amendment.

The parties, intending to be legally bound, hereby further agree:

This Amendment has been duly authorized, executed and delivered by, and is a valid and binding agreement of, the Company, enforceable in accordance with its terms, except as the enforcement hereof may be limited by bankruptcy, insolvency, reorganization, moratorium or other similar laws relating to or affecting the rights and remedies of creditors or by general equitable principles.

This Amendment and any claim, controversy or dispute arising under or related to this Amendment shall be governed by and construed in accordance with the internal laws of the State of New York applicable to agreements made and to be performed in such state. Any legal suit, action or proceeding arising out of or based upon this Amendment or the transactions contemplated hereby or by the Original Agreement may be instituted in the federal courts of the United States of America located in the Borough of Manhattan in the City of New York or the courts of the State of New York in each case located in the Borough of Manhattan in the City of New York (collectively, the “Specified Courts”), and each party irrevocably submits to the exclusive jurisdiction (except for proceedings instituted in regard to the enforcement of a judgment of any such court, as to which such jurisdiction is non-exclusive) of such courts in any such suit, action or proceeding. Service of any process, summons, notice or document by mail to such party’s address set forth in the Original Agreement, as amended by this Amendment, shall be effective service of process for any suit, action or other proceeding brought in any such court. The parties irrevocably and unconditionally waive any objection to the laying of venue of any suit, action or other proceeding in the Specified Courts and irrevocably and unconditionally waive and agree not to plead or claim in any such court that any such suit, action or other proceeding brought in any such court has been brought in an inconvenient forum.

The Original Agreement, as amended by this Amendment, supersedes all prior written or oral and all contemporaneous oral agreements, understandings and negotiations with respect to the subject matter hereof. This Amendment may not be amended or modified unless in writing by all of the parties hereto, and no condition herein (express or implied) may be waived unless waived in writing by each party whom the condition is meant to benefit.

The invalidity or unenforceability of any section, paragraph or provision of this Amendment shall not affect the validity or enforceability of any other section, paragraph or provision hereof. If any section, paragraph or provision of this Amendment is for any reason determined to be invalid or unenforceable, there shall be deemed to be made such minor changes (and only such minor changes) as are necessary to make it valid and enforceable.

The Company and the Agents each hereby waive any right to trial by jury in any suit or proceeding arising under or related to this Amendment.

This Amendment may be executed in counterparts, each of which shall be deemed an original, but all such respective counterparts shall together constitute one and the same instrument. This Amendment may be delivered by any party by facsimile, email or other electronic transmission.

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If the foregoing correctly sets forth the understanding among the Company and each Agent, please so indicate in the space provided below for that purpose, whereupon this letter shall constitute a binding amendment to the Original Agreement between the Company and each Agent.

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