Not all cold climates lack sun. Here at 6-10,000' in northern New Mexico, we still get lots of sun in the winter, but also very cold (overnight) temperatures. Air source heat pumps make more sense in almost every way, and will operate down to -13F/-25C.

Our 6.6kW array can provide roughly 1/3 the electricity we need to heat all of our 1875 adobe home, which is poorly insulated at best. If this was closer to passivhaus standards, I imagine it could get close to 100%.