https://www.irs.gov/newsroom/faqs-for-modification-of-sections-25c-25d-25e-30c-30d-45l-45w-and-179d-...
The Code says an expenditure is treated as occurring when installation is "completed." The form 5695 instructions also say "completed". I tried to look for a more comprehensive definition of what "completed" means and I couldn't find one.
Usually, folks on this board will say that "completed" includes final inspection and turn-on. However, it might be worth it to claim the credit even if the inspections aren't done, knowing that there could be a gray area if you are audited. (For business credits, the term is "placed in service", which I would argue definitely includes being inspected and turned on. But "placed in service" is not the language used for the residential credit.)
