If your replacement windows meet certain criteria, you can file IRS Form 5695 to obtain a residential energy efficiency tax credit. The form and accompanying instruction booklet are located at IRS.gov.
As of 2015, in order for your replacement windows to qualify for the residential energy-efficiency tax credit, they must be Energy Star-qualified replacement windows that were installed between Jan. 1, 2012 and Dec. 31, 2014 to improve an existing home, not a new home. Some manufacturers offer a Manufacturer's Certification Statement intended to corroborate a product's Energy Star qualification and its eligibility for tax credits as explained within the Internal Revenue Code, Section 25C.
Submit the credit with your taxes for the year the windows were installed. You cannot claim more than $500 total in residential energy-efficiency tax credits and no more than $200 for eligible windows. Keep all of your receipts and proofs of purchase for qualifying products, and save the National Fenestration Rating Council, or NFRC, label for each window product. You may need these in the event of an audit.
Visit EnergyStar.gov to obtain up-to-date information about Energy Star program requirements and eligibility for residential energy-efficiency tax credits. Eligible products also include those manufactured by an Energy Star partner, independently tested and certified by the NFRC, or products that have NFRC ratings that meet U.S. Department of Energy guidelines.