EXHIBIT 5.1

New York
Menlo Park
Washington DC
London
Paris

Madrid
Tokyo
Beijing
Hong Kong

Davis Polk & Wardwell LLP
450 Lexington Avenue
New York, NY 10017

212 450 4000 tel
212 701 5800 fax

October 22, 2010

First Wind Holdings Inc.
179 Lincoln Street, Suite 500
Boston, Massachusetts 02111

Ladies and Gentlemen:

First Wind Holdings Inc., a Delaware corporation (the “Company”), is filing with the Securities and Exchange Commission a Registration Statement on Form S-1 (the “Registration Statement”) for the purpose of registering under the Securities Act of 1933, as amended (the “Securities Act”), 13,800,000 shares of its Class A common stock, par value $0.001 per share (the “Securities”), including 1,800,000 shares subject to the underwriters’ over-allotment option, as described in the Registration Statement.

We, as your counsel, have examined such documents and such matters of fact and law that we have deemed necessary for the purpose of rendering the opinion expressed herein. Based on the foregoing, we advise you that, in our opinion, when the price at which the Securities to be sold has been approved by or on behalf of the Board of Directors of the Company and when the Securities have been duly issued and delivered against payment therefor in accordance with the terms of the Underwriting Agreement referred to in the Prospectus which is a part of the Registration Statement, the Securities will be validly issued, fully paid and non-assessable.

We are members of the Bar of the State of New York and the foregoing opinion is limited to the General Corporation Law of the State of Delaware.

We hereby consent to the filing of this opinion as an Exhibit to the Registration Statement and further consent to the reference to our name under the caption “Legal Matters” in the Prospectus which is a part of the Registration Statement. In giving this consent, we do not admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act.

Very truly yours,

/s/ Davis Polk & Wardwell LLP

First Wind Holdings Inc. S-1/A