This question is sort of like my own, except that my system is still on the drawing board.
I want to preheat the incoming cold water with a solar tank, and have the electric heater downline from that.
I will build a ‘coffin’ at ground level, insulated and with reflective interior. In that I’ll put a recycled waterheater from a scrap pile and plumb it in a straight line, input through the tank, to output, to the heater.
Any ideas as to the problems and/or wisdom of doing that?