I’ll review the last show on solar hot water systems.   A system that is designed to heat the hot water only has a different angle to the sun than one that is used to heat the house as well.  Summer hot water use is only slightly less than the winters, so year round collection is needed and a lower angle is appropriate.  The backup for hot water can be small enough that an electric element can supply enough heat to supplement the solar when needed, without using much electricity.  Electric resistance backup for home heating would require a larger heating element and would run longer than for the hot water.