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                                                                     EXHIBIT 5.1

                                 August 20, 2002

Environmental Power Corporation
One Cate Street, 4/th/ Floor
Portsmouth, New Hampshire 03801

         Re:    Registration Statement on Form S-8

Ladies and Gentlemen:

         We have acted as counsel to Environmental Power Corporation, a Delaware
corporation (the "Company"), in connection with the registration under the
Securities Act of 1933, as amended, on a registration statement on Form S-8 (the
"Registration Statement") of (i) an aggregate of 3,000,000 shares of the
Company's common stock, $0.01 par value per share (the "Incentive Shares"),
which are to be offered and sold to employees, officers, consultants,
independent contractors and non-employee directors under the Company's 2001
Stock Incentive Plan (the "Incentive Plan"), (ii) an aggregate of 2,000,000
shares of the Company's common stock (the "Director Shares"), which are issuable
upon the exercise of options (the "Director Options") to purchase shares of
common stock which may have been granted and which may be granted to outside
directors of the Company under the Company's 2002 Director Option Plan (the
"Director Plan"), and (iii) an aggregate of 450,000 shares of the Company's
common stock (the "Weisberg Shares" and, together with the Director Shares, the
"Option Shares"), which are issuable upon the exercise of options (the "Weisberg
Options" and, together with the Director Options, the "Options") to purchase
shares of common stock which were granted to Robert I. Weisberg pursuant to
certain Stock Option Agreements (the "Weisberg Agreements").

         We have examined such documents and reviewed such questions of law as
we have considered necessary and appropriate for the purposes of our opinion. In
conducting such examination, we have assumed (i) that all signatures are
genuine, (ii) that all documents and instruments submitted to us as copies
conform with the originals and (iii) the due execution and delivery of all
documents where due execution and delivery are a prerequisite to the
effectiveness thereof. As to any facts material to this opinion, we have relied
upon statements and representations of officers and other representatives of the
Company and certificates of public officials and have not independently verified
such facts.

         Members of our firm are admitted to the Bar of the State of New York,
and we do not express any opinion as to the laws of any jurisdiction other than
the Delaware General Corporation Law.

         Based on the foregoing, we are of the opinion that (i) the Option
Shares issuable upon exercise of the Options issued or issuable under the
Director Plan and the Weisberg Agreements will be validly issued, fully paid and
nonassessable when issued in accordance with the Director Plan and the Weisberg
Agreements, respectively, and (ii) the Incentive Shares have been duly
authorized and, upon issuance, delivery and payment thereof in accordance with
the terms of the Incentive Plan, will be validly issued, fully paid and
nonassessable.

         We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement. The foregoing opinion is being furnished to you solely
for your benefit and may not be relied upon by, nor may copies be delivered to,
any other person without our prior written consent.

                                Very truly yours,
                                /s/ Dorsey & Whitney

