It's true that flat panel solar panels, which are easy to build your own, don't work at all on overcast or cloudy days. But if you look at the price of the evacuated tubes, and consider the amount of them to equal my 4 by 32 foot flat panel, you see why compromising on cloudy days is not a bad compromise. My homebuilt EPDM lined 800 gallon insulated tank lets me ride through cloudy days, even in winter. Domestic hot water use has no appreciable impact on my hot water storage temp. Heating the house does!