Good point. It's unclear to me though. In some places (Hawaii, California, Germany) where there's huge acceptance of renewables and much shorter payback periods I believe a PV system would add value to a sale. Here in Michigan it's much less clear - plenty of my neighbors think it's ugly and an eyesore, or are somehow opposed to the whole concept. So I am pretty sure in the end it's a net zero here (no pun intended!)
If that's the case, the system itself wouldn't be a non-refundable investment, and calculating ROI on it would be as silly as calculating ROI on your house.