The stupidest use of Photovoltaic Panels was an installation I saw that used two panels, approximately 1.25 meter by 2 meter to power a resistance hot water heater used to heat a quantity of water. Why would one waste PV panels in this way when a direct heat exchanger is so much more efficient? Am I missing something?
What about using both PV cells and heat exchangers? A solar cell is only something like 20% efficient and works better at lower temperatures anyway so it could be cooled by water which is heated for home use.
Ah-ha! They use the PVs so it will still work when it's cold, I didn't consider that. However, I agree that both would be better - use the heat exchanger above temp "X," (whatever that turns out to be for best balance) while storing electricity for other uses. Only use the stored energy to heat the water when it becomes too cold for the heat exchangers to work.
Practically, since "grid power" is available at that location, the most economic installation probably would be heat exchangers only, and use an electric heater when necessary, but that wouldn't have the "feel good" factor. Somehow, "reducing grid usage by 80-90 percent" isn't good enough, and "100 percent off grid" is better, even though the cost, overall, is worse. (those cells could be put to better use elsewhere.)