I’ve said this before in other threads, but I think what it really comes down to is the fact that the IRS guidelines are somewhat ambiguous and different people interpret them differently. There really is no black and white yes or no answer here. In fact, tax returns are basically done on the honor system. Anyone could claim credit for a solar system installed this year, whether they actually had one installed or not and unless they got audited no one would ever know.
If you are claiming the credit you (and/or your tax professional) needs to understand the IRS guidelines and be able to defend your choices in the event of an audit. An arguement like
@Kodemonkey made above sounds like a great defense. Saying “Well, a bunch of guys on the internet said it was ok” is probably not so great.
So in the end, there really isn’t any definitive answer here. If you wind up in the position where your system has been installed, but you don’t have PTO yet you need to read and understand the IRS guidelines such that you are comfortable being able to defend your reasons for taking the credit in case you do get audited.