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 )

Opinions ???
What kind of pipe ?
Would I need to fasten it to the rooftop ??
rj