[jabl]

That's true, but a non-trivial factor driving down the cost of solar electricity is not only the panels themselves, but economy of scale in all the rest, like electrical systems to connect them to the grid, support structures for the panels, installation and maintenance, etc. etc. So rooftop solar, while attractive from the perspective of not industrializing the wilderness, is more expensive than utility-scale solar.

But yes, it would be nice with building codes requiring to either have solar or green roofs. That would certainly help drive down costs, and in the case of green roofs, reduce cooling demands as well as providing (depending on what you plant, granted) a haven for biodiversity.