26 U.S.C. Section 48 Legislation authorizing Investment Tax Credit for renewable energy facilities. 

• Form 3468, Investment Credit IRS Form 3468 to claim Investment Tax Credit

26 U.S.C. Section 45 Legislation authorizing Production Tax Credit from renewable energy sources.

• General Instructions for claiming credit.
• IRS Overview of Advanced Energy Credit for Manufacturers

The Energy Improvement and Extension Act of 2008 (PDF) Division B, Public Law 110-343, 122 STAT. 3807

1603 Program: Payments for Specified Energy Property in Lieu of Tax Credits

Election of Investment Tax Credit in Lieu of Production Tax Credit Coordination With Department of Treasury Grants for Specified Energy Property in Lieu of Tax Credits