You can only claim the solar credit for a home that you use as a residence. It doesn't have to be your main residence, but it must be a residence. So if you have (for example) a main house and a mother-in-law cottage, you could only claim the credit for any system that feeds the main house, unless you live in the cottage for at least part of the year.
The system must provide power to your residence, so a system on a shed that powers the house is eligible, but a system that powers a greenhouse is not eligible unless it is cross-connected to the house and feeds the house. So that might create a kind of loophole, if you put panels on your house and on the mother in law cottage but they were all connected as one linked system that feeds the entire property, that might be allowable.
If you install qualifying property twice, both installations would be eligible, and the credit is claimed based on the date when it is inspected and turned on, not when you paid or signed the contract.
