Heating hot water with roof mounted panels is very expensive. Your going to spend a few grand to save a couple of hundred a year. So that means your payback is ten plus years down the road. Well guess what, these systems need repair from time to time, so the pay back gets even farther down the road. On top of that, when it comes time to re-roof the house or repair the roof, the panel has to come down. Guess what, most times the panels do not go back up, because it costs too much.
I installed a system on my house in 1996. Found it used for $100.00. Had to rebuild the collector, all the pipes were split. Moved the storage tank due to a remodel. Had the controller fixed once for $50.00. It's still working, saving me like $100.00 a year in natural gas. It probably cost me $800.00 in my labor so far. My roof needs to be replaced, so the panel has to come down. That's another couple of hundred to do that. I'm money ahead, but there is no big savings, not unless your using very expensive energy to heat your water.
Plus, you have to remember, the original owner of the house that installed the system, may no longer own the house. The new owners may care less about a system on their roof, they may even hate the look of it. So they remove it at some point. Such as when it breaks, or the roof has to be done or it's just a eye sore to them.
I expect the same thing to happen to the photovoltaic systems, for pretty much all the same reasons. Inverter goes out, trees shade it or it gets damaged from a falling branch. Roof needs fixing or another roof, so off comes the system, for good...