anyone built solar panals for heating water my father waants to try it

The hot water in my dads house is mostly heated by the sun. When he built the house, he designed a small greenhouse to hold 2 water tanks. It gets very hot in there on sunny days. The two tanks he put in there he painted black. We still have a conventional hot water heater, and the two systems are integrated. The hot water heater doesn't need to work that much (or at all) if the water from the black tanks is already hot, thus conserving energy. But, if there are some cloudy days then we have the hot water heater to fall back on.