I'll try to give you a link below, but this forum often censors links, so if the link doesn't work, you can Google "Tax Code IRC 25D".
That section (25D) of the Tax Code gives a credit for an "expenditure" for solar and other things. Then under the special rules, (e)(8) says when the "expenditure" is considered as made.
(8) When expenditure made; amount of expenditure
