Assuming you were living in the home when the installation occurred, the deciding factor is "when the original installation of the item is completed." It doesn't matter when you paid, when you signed the contract, when you got permission to operate from your utility, or when the solar was "placed in service."

All that matters in terms of determining the taxable year to claim the tax credit is "when the original installation of the item is completed." So if the installation of the solar was completed in 2023, you must claim it on your 2023 taxes, which you typically would file in April 2024.

Here's the details with citations to the law: https://www.reddit.com/r/solar/comments/yo7g32/can_i_get_the_tax_credit_this_year_if_still/ivdsugt/?context=3

Also, for some reason, whenever I tell people about the law, someone else inevitably comes along and is quite adamant that the test is when the system is "placed in service." Since I really don't want to get into a debate over this issue again, especially because another comment already said "in service," I provide the following quotes from the IRS:

Form 5695 instructions:

>costs are treated as being paid when the original installation of the item is completed,

IRS FS-2022-40, page 6:

>Q1. May a taxpayer claim the credits in the year of purchase if installation of the qualifying property occurs in a later year? (added December 22, 2022)
>
>A1. No. A taxpayer may not claim the credits until the year the property is installed.

IRS 2013-70, Q-24/A-24:

>Q-24: In 2010, Taxpayer A purchases and moves into a newly constructed home that contains qualifying § 25D property. Taxpayer A claims the § 25D credit in 2010. In 2013, Taxpayer A sells the home to Taxpayer B. Is Taxpayer B eligible for a credit on the same § 25D property?
>
>A-24: No. Section 25D(e)(8) generally treats an expenditure as made when the original installation of the qualifying property is completed. . . . .

IRS Private Letter Ruling 201809003:

>Under § 25D(e)(8)(A) of the Code, generally, for purposes of determining the tax year when the credit is allowed, an expenditure with respect to an item shall be treated as made when the original installation of the item is completed.

IRS Private Letter Ruling 201536017:

>Under § 25D(e)(8)(A), generally, for purposes of determining the tax year when the credit is allowed, an expenditure with respect to an item shall be treated as made when the original installation of the item is completed.