Awhile back I saw this simple solar hot water heater made of a recycled Refrigerator Coil.
It basically had a plastic tank of water positioned on a higher elevation with two vinyl tubes
coming out of the side. One higher than the other.
The solar panel wasn't much larger than an old fashioned wash board so the coil must have
came from a small Fridge. (one of those Dormitory Fridge's)
The tubing at the bottom of the plastic tank connected to the bottom of the coil of the solar
panel. The one at the top to the upper connection of the coil of the solar panel.
During the day time it could provide hot water for various needs. Just to have ready hot water.
I'm sure this idea could be refined to have a domestic water coil built into the same tank so that
the drinking water would be free of any contamination from the Fridge coil. (if that's an issue)
It would only require a small 12 VDC pump to send cold water from a small tank through the domestic coil and deliver hot water to where it's needed. (like an on demand hot water heater)