In general, I would include all costs associated with upgrading the roof and house and shingles necessary to install your solar panels. It might be a stretch to include walls and foundation etc. But if you install solar panels on a thatch roof you are defeating Congress's intent of both replacing legacy electricity with solar and generating employment which eventually pays roughly 30% in taxes, which is what you are not paying.
And the IRS doesnt look at ten houses and say three need a new roof, three can use the old roof, and one only needs 684 shingles updated to support 18 solar panels. They may look at a letter from a roofer that says not to put 40 solar panels on a 23 year old beat up roof, like my house, but they are more interested in verifying with documentation that you paid a roofing company to replace your roof and you didnt do it yourself at no labor cost and worse with free or stolen shingles but filled out the form based on the value of the new roof.
Comments have mentioned that nothing stops someone from building a house and claiming the entire cost for the tax credit. The instructions do talk about the structural parts needed to support the panels, but the real limitation is that you can only offset the tax you owe. In 2020, if you only make enough to owe $20K in taxes, you can only offset $20k in tax credits. You can carry over a balance in 2021 and by 2022 this goes away, unless it is renewed. One thing is certain: Uncle Sam is not rebating more than you gave him. And most likely, those Powerwalls and roofers, and electricans and their employers, are paying about 26% of the money you gave them, in taxes, probably more.
I want to hear more people explain why they think Congress only wants solar shingles that act as roofs to qualify for the credit when solar shingles were still (and practically speaking, are still) in the R&D phase when the credit was first passed.
