When I run the numbers, a solar water heater does not work out for me. In order for me to eliminate my electric heater I would need an active (somewhat more costly) solar heating system which run ~$4k the lower end if you include the cost of installation. At a rate of about $500 a year in water heating costs that's a 8 year payback timeframe for a device that has a 5 year warranty (more expensive models have longer warranties).
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.
The best time to do an install is when you’re looking at buying a water heater either way. Thus significantly reducing the price premium. Further, sizing a system to also handle home heating is generally where the real savings takes place.
As to warranty, they generally last well past that point. You may be replacing a part or to at ~10 years, but a 20+ year lifespan is common depending on the model and your water hardness.
