Sunbelt state. With that said, I have helped install rooftop solar on roofs as far north as Chicago where, when properly insulated, the house can still use a heat pump for heating needs even in the middle of winter (down to -30F) and supports one of the residents commuting 100 miles/day in their Model S. They still have gas backup though.
Keep in mind, the top three states by population are California, Texas, and Florida. Heat pumps can meet the needs of housing in these climates, alongside rooftop solar. Produce where you consume when able.
