The problem is that a BTU of natural gas currently costs about 1/5 what a BTU of electricity costs (depending on the efficiency of the gas water heater). That's down from about 1/3 a few years ago. So replacing an electric water heater with a solar one pays back 5x sooner than replacing a natural gas water heater with a solar one.I too like solar, but on a smaller scale like residential water heating.
Each solar BTU collected saves a fossil fuel BTU . Whatever you collect you don't have to buy from electric company.
Here's a plant built by an outfit in Florida. It preheats water for a steam power plant.
There's some uncomfortable chemistry involved and as the NYT article below mentions it's really not good economics.
But , every solar BTU they collect is one they don't have to buy from the natural gas company.