I have a PV panel that runs my 12 volt circulator pump on my SHW system. Some folks just hook the PV to a 12 volt pump with no controls, that works but has some issues in the winter and cold mornings and in the afternoon, so I have a fairly simple controller that looks at the temp at the top of the panel and only runs the pump when the top of the panel is hotter than the tank. So even with the power out I will end up with a tank of hot water but unless I want to hook a hose its not of much use. The SHW system went in before the PV so at the time it made sense. as it saved the electrical use of running a circ pump. Given the HPHW heaters are so inexpensive, I wouldn't install a SHW system and would just install a few more PV panels to run the HPHW heater but since its installed I use it.