Exhibit 5.1

ATTORNEYS AT LAW

111 Huntington Avenue

Boston, Massachusetts 02199

617.342.4000 TEL

617.342.4001 FAX

www.foley.com

October 13, 2023

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 preparation of a Registration Statement on Form S-3 (the “Registration Statement”), including the prospectus constituting a part thereof (the “Prospectus”), filed on the date hereof by the Company with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Securities Act”). The Registration Statement relates to the issuance and sale by the Company from time to time, pursuant to Rule 415 of the General Rules and Regulations of the Commission promulgated under the Securities Act, of an indeterminate amount of shares of the Company’s common stock, par value $0.0001 per share (the “Common Stock”). The Prospectus provides that it will be supplemented in the future by one or more supplements to such Prospectus and/or other offering material (each, a “Prospectus Supplement”).

In connection with our representation, we have examined: (a) the Registration Statement, including the Prospectus and the exhibits (including those incorporated by reference) constituting a part of the Registration Statement; (b) the Certificate of Incorporation of the Company, as amended to date; (c) the Second Amended and Restated By-laws of the Company, as amended to date; (d) resolutions of the Company’s Board of Directors relating to the filing of the Registration Statement and the sale of the Common Stock covered by the Registration Statement; and (e) such other proceedings, documents and records as we have deemed necessary to enable us to render this opinion.

In our examination of the above-referenced documents, we have assumed the genuineness of all signatures, the authenticity of all documents, certificates and instruments submitted to us as originals and the conformity with the originals of all documents, certificates and instruments submitted to us as copies. We have also assumed that (i) the Registration Statement, and any amendments thereto (including post-effective amendments), will have become effective and will comply with all applicable laws; (ii) a Prospectus Supplement, if required, will have been prepared and filed with the Commission describing the offering; (iii) all shares of Common Stock will be issued and sold in compliance with applicable securities laws and in the manner stated in the Registration Statement and any applicable Prospectus Supplement; (iv) a definitive purchase, underwriting or similar agreement, if any, with respect to any shares of Common Stock offered will have been duly authorized and validly executed and delivered by the Company and the other parties thereto; and (v) with respect to shares of Common Stock offered, there will be sufficient shares of Common Stock authorized under the Company’s Certificate of Incorporation, as amended, and not otherwise reserved for issuance.

AUSTIN

Boston

CHICAGO

dallas

DENVER

DETROIT

houston

JACKSONVILLE

LOS ANGELES

MADISON

MEXICO CITY

MIAMI

MILWAUKEE

NEW YORK

ORLANDO

SACRAMENTO

salt lake city

SAN DIEGO

SAN FRANCISCO

SILICON VALLEY

TALLAHASSEE

TAMPA

WASHINGTON, D.C.

BRUSSELS

TOKYO

October 13, 2023

Page 2

Based upon and subject to the foregoing, we are of the opinion that:

1.The Company is a corporation validly existing and in good standing under the laws of the State of Delaware.

2.All requisite action necessary to make any shares of Common Stock offered by the Company validly issued, fully paid and nonassessable will have been taken when:

a.The Company’s Board of Directors, or a committee thereof duly authorized by the Board of Directors, shall have adopted appropriate resolutions to authorize the issuance and sale of the Common Stock; and

b.Such shares of Common Stock shall have been issued and sold for the consideration contemplated by, and otherwise in conformity with, the Registration Statement, as supplemented by a Prospectus Supplement with respect to such issuance and sale, and the acts, proceedings and documents referred to above.

We express no opinion as to the laws of any jurisdiction other than the State of Delaware and the federal laws of the United States.

This opinion is being furnished to you for submission to the Commission as an exhibit to the Registration Statement in accordance with the requirements of Item 16 of Form S-3 and Item 601(b)(5) of Regulation S-K promulgated under the Securities Act. We hereby consent to the filing of this opinion as an exhibit to the Registration Statement and to the reference to our firm under the caption “Legal Matters” in the Prospectus forming a part thereof. 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.

	Very truly yours,

	/s/ FOLEY & LARDNER LLP