Yeah, but incident solar radiation is rarely a large component of the energy used to heat a home. If it were, we could just live in greenhouses and forget the whole fireplace vs. gas vs. oil vs. electric vs. heat pump debate. You get a much bigger boost to heating efficiency by having new windows installed or cleaning your ductwork than by keeping your roof sunny.

Also, heat pumps can be up to 400% efficient, so the electrical energy that comes from a solar panel could actually turn a profit thermally if used to run a heat pump.