@LauraKuhs - once you've installed the solar, you've earned the credit based on the year the equipment was installed. It does not matter that the future year is a lower tax credit.
THEN, the issue is whether you can take the credit or have to roll it over until future years - you never lose it.
Look at line 22 on your 2021 Form 1040 - THAT line can never go below zero, so the solar tax credit can reduce Line 22, but not below sero. Whtever is left over is the carryover to a future year.
