farms?

Yes. I'm sure that will be the way they go in the end. But businesses want to do it as cheap as possible. Make a quick buck for the lowest possible costs as usual. On roofs is a little more complex. Legislation is probably needed.

You do have to be careful though, sometimes those tarpaper roofs are trying to keep heat inside the building, or trying to keep lines from freezing in the winter.

That being said, there's plenty of places in Japan where you could throw up a canopy of solar panels tied into the grid.