There are several different credits and you are getting them confused.
The Energy Efficient Home Improvement Credit covers improvements to make your home more energy efficient, like doors, windows, insulation, efficient furnace, air conditioning, and so on. This credit has a $1200 annual cap, as well as several sub-caps ($600 for energy property, $600 for windows, $500 for doors). Separate from the general $1200 cap, there is a $2000 cap for installation of an electric or natural gas heat pump water heater, an electric or natural gas heat pump, or a biomass stove or boiler.
Then, there is an entirely different credit called the Residential Clean Energy Credit. This includes solar electric, solar water heating, fuel cell, small wind energy, geothermal, and battery storage. The credit is 30% with no maximum except fuel cells ($500 per half kilowatt) and you can claim the credit in any year you install qualifying property.
