Can you do solar preheat as a retrofit to an existing electric (or natural gas) hot water heater? Coil some flexible water line in a plywood box, spray paint it all black, put a plexiglass lid on, caulk it up, and lay it out in the sun. Feed cold water through it before it hits your hot water heater.
It probably would not be as efficient as a commercial solution, but it would be very cheap.
Don't forget about water safety, backflow valves, or local plumbing and building codes. Or winter - would want a bypass valve for flushing and draining it.
I think you are right and there are gains to be had with a hybrid system. I’m handy with plumbing and I think it is something I could figure out but the problem is, as always for everyone, I have so many other things going on I that don’t have the time to figure out a DIY system.
It probably would not be as efficient as a commercial solution, but it would be very cheap.
Don't forget about water safety, backflow valves, or local plumbing and building codes. Or winter - would want a bypass valve for flushing and draining it.