We always say the credit is claimed as of the date they are "placed in service," regardless of the payment arrangements. Typically, this means they are installed, inspected, and providing power. So that would be on your 2023 tax return, based on your facts.
If you have entered an arrangement where you are supposed to pay the incentive to them as a downpayment, but you won't actually get the incentive until 2024 due to their delay in turning the system on, I think you need to have a conversation with the sales department. Here's the actual text of the law.
https://www.law.cornell.edu/uscode/text/26/25D
Footnote: The actual law says "Except as provided in subparagraph (B), an expenditure with respect to an item shall be treated as made when the original installation of the item is completed." However, when the law talks about the declining credit percentage it says "in the case of property placed in service after December 31, 2021, and before January 1, 2033, 30 percent; in the case of property placed in service after December 31, 2032, and before January 1, 2034, 26 percent" and so on. If "completed" did not mean the same thing as "placed in service", then you would have a situation where something completed in December 2032 but not turned on until January 2033, would qualify for a 26% credit to be claimed on the 2032 return where the credit is 30%. Laws are usually read as if they make sense, so most likely a court would rule that in this case "completed" and "placed in service" are the same thing.
