@SteamTrain --
You're right, the residential energy credit was extended though 12/31/2020. For qualified windows and doors, the taxpayer can take a credit of 10% of the cost (not including installation) up to $500 (windows are capped at $200).
Equipment Tax Credits for Primary Residences | About ENERGY STAR | ENERGY STAR
I think a sliding glass door would qualify as a "door" rather than a window.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
