I am also not a lawyer nor CPA / tax attorney, so just like
@jboy210 said, I am just "a guy on the internets" but what I found agrees with the other posters on this thread. Saying that, I think the one thing I thing we all agree on is that its not "when construction starts" because that is under the commercial part of the instructions. Nor can you pay for parts early to qualify.
Lets use an example. I have "A friend" (yeah thats it.. a friend.. not me at all...) who had an install date originally of december 18th or 19th of 2019, something like that. This friend of mine did a bunch of looking into this to see would he be able to take the credit in 2019 or would he have to wait until 2020.. since PTO definitely would not come until 2020.
Tesla informed m.....err him (

) that the install date was going to be moved to January 6th 2020. He dug just a little deeper to see if he could pay tesla in full for the system in 2019 so that he could claim it on 2019s taxes because thats when he wanted to take the credit. Unfortunately, he found out that, although there are stipulations for payments to count toward system installation, they did not apply to residential installs.
So, he was unable to pay early (it wouldnt have mattered anyway) and so unable to take the credit in 2019, and will have to do it in 2020. He also missed the 30% credit and will get 26% but got tesla to refund him the difference because the issue with the install date being missed was a tesla mistake.