Solar water heating system..
If a 4'x12' tank cover collects 90% of the sun that enters the glazing (91.4K Btu, with the help of 2'x12' of Big Fins on the south tank wall) in Tw F water and loses 6h(Tw-Tg)1.5x6x12 = 648(Tw-Tg), Q = 305(352-Tw). Tw = 140 F makes Q = 64.5K Btu (19 kWh) per day, ...