The law says that the taxable year to claim the tax credit is the year "when the original installation of the item is completed."

Accordingly, if the solar is completed being installed in 2022, the taxpayer would claim the tax credit for the entire cost of the solar system on his or her 2022 taxes, which typically would be filed in April 2023. Here's a comment I made a few months ago that gives more details: https://www.reddit.com/r/solar/comments/yo7g32/can_i_get_the_tax_credit_this_year_if_still/ivdsugt/?context=3