Exhibit 5.1

EXHIBITS 5.1 AND 23.1

February 4, 2015

Pattern Energy Group Inc.

Pier 1, Bay 3

San Francisco, California 94111

Ladies and Gentlemen:

We have acted as special counsel for Pattern Energy Group Inc., a Delaware corporation (the “Company”), in connection with (i) the Registration Statement on Form S-3 (the “Registration Statement”) filed with the Securities and Exchange Commission pursuant to the Securities Act of 1933, as amended (the “Securities Act”) for the purpose of registering under the Securities Act certain securities, including shares of its Class A common stock, par value $0.01 per share (“Class A Common Stock”) and (ii) the prospectus included in the Registration Statement (the “Basic Prospectus”), as supplemented by the prospectus supplement dated February 3, 2015 (the “Prospectus Supplement,” and together with the Basic Prospectus, the “Prospectus”) relating to the offering of up to 13,800,000 shares of Class A Common Stock, including up to 7,000,000 shares of Class A Common Stock (the “Primary Shares”) to be sold by the Company and up to 6,800,000 shares of Common Stock (the “Secondary Shares,” and together with the Primary Shares, the “Shares”) to be sold by the selling stockholder named in the Prospectus (the “Selling Stockholder”). The Shares include up to 1,800,000 shares of Common Stock subject to the underwriters’ over-allotment option, as described in the Prospectus.

We, as your counsel, have examined originals or copies of such documents, corporate records, certificates of public officials and other instruments as we have deemed necessary or advisable for the purpose of rendering this opinion.

In rendering the opinion expressed herein, we have, without independent inquiry or investigation, assumed that (i) all documents submitted to us as originals are authentic and complete, (ii) all documents submitted to us as copies conform to authentic, complete originals, (iii) all signatures on all documents that we reviewed are genuine, (iv) all natural persons executing documents had and have the legal capacity to do so, (v) all statements in certificates of public officials and officers of the Company that we reviewed were and are accurate and (vi) all representations made by the Company as to matters of fact in the documents that we reviewed were and are accurate.

Based on the foregoing, we advise you that, in our opinion:

In connection with the opinion expressed above, we have assumed that the Company is validly existing as a corporation in good standing under the laws of the State of Delaware.

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

We hereby consent to the filing of this opinion as an exhibit to a report on Form 8-K to be filed by the Company on the date hereof and its incorporation by reference into the Registration Statement and further consent to the reference to our name under the caption “Validity of Securities” in the prospectus supplement, 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