Look at line 24 of your 2021 form 1040. That is your total tax liability for the tax year.
Now look at line 33 of the form 1040 for tax year 2021. That is the total amount of taxes you paid during the year through W-2 withholding, quarterly tax payments, etc. If the amount on line 33 is more than line 24, then you over paid your tax liability and the difference is refunded to you.
The solar tax credit affects your tax liability on line 24. If your total possible tax credit for solar panels is $8000, but your tax liability is only $5000, the credit will only reduce your liability to zero - meaning that you will only get a $5000 credit. That's it. Not a penny more.
Now with some credits any "excess" you may qualify for can be carried forward to the next tax year. I do not know if the solar tax credit is carried forward. But if it's not, then the maximum credit you can possibly get, can not exceed your tax liability in the tax year you claim the credit.
