The first part is the best advice for everybody asking since none of us will stand behind somebody else's tax return in the event of an IRS audit.
That said, the way I have read it, I agree that install complete is all that is necessary - not inspection and not PTO. If Tesla installs, and ideally does at least a brief test (just to have some documentation that even 0.1 kWh was generated) that should suffice. Even payment may not be required to claim the 26% (though I note it might be interesting to see the 2020 forms and whether it indicates you could claim the 26% but would have to wait a year to claim it.) But, since this issue did not apply to me, I can't be certain of all the rules.