We get LOTS of sunshine in Southern Arizona.
Without getting too elaborate, here's my idea;
Loop about 100' of plastic pipe in the rooftop.
Connect FROM cold inlet TO my water heater.
During the daytime, ( which is when we use hot water )
Water would be solar-preheated before going to the water heater.
No need for any "controls" except to disconnect it
in the wintertime. ( temps do dip to freezing at nite in mid-winter )
What kind of pipe ?
Would I need to fasten it to the rooftop ??