As per your link, solar thermal water heating is very variable depending on geography: > The amount of heat delivered by a solar water heating system depends p

As per your link, solar thermal water heating is very variable depending on geography: > The amount of heat delivered by a solar water heating system depends primarily on the amount of heat delivered by the sun at a particular place (insolation). In the tropics insolation can be relatively high, e.g. 7 kWh/m2 per day, versus e.g., 3.2 kWh/m2 per day in temperate areas. Because of this, many solar thermal systems are supplemented by convention heaters. > In winter, the percentage of your hot water heated by the sun drops to as low as 10-20%—as you might expect with short days and weak sun in December. That’s why practically every solar water installed in the US will be connected to a backup conventional water heater to ensure that your hot water needs continue to be met even in January. https://www.aspessolarproducts.com/solar-water-heater/how-does-a-solar-water-heater-work-in-winter.html https://www.aspessolarproducts.com/solar-water-heater/how-do...