with some shenanigans and thermally massive sinks (say boilers) heating can be intermittently fed, i don’t think a lot of people (some industries might) are bothering with that for now though. (so not heat pumps but say overtly massive boiler with radiator and a fan attached to central hvac system to provide heat as needed, and heat pump as a backstop). plus winter solar efficiency by default is what 40-60%?