You can use otherwise unused space - that's the theory. Now if you install solar heating panels on your roof you can actually heat your house and warm your wate

You can use otherwise unused space - that's the theory. Now if you install solar heating panels on your roof you can actually heat your house and warm your water with that, even in winter. They're fairly efficient. Nobody does that in germany though, since it's more economical to sell the power generated by solar panels.