I agree with Don.
The credit is for the "expenditure" for solar.
The "expenditure" "shall be treated as made when the original installation of the item is completed" (IRC 25D(e)(8)).
In my opinion, the installation can be completed even if the utility company hasn't told you that you can turn the power switch "on". My installer even showed me that my meter was running backwards when he tested it, but then just told me not to turn the switches back "on" until the utility company came out. To me, the installation was still completed.
