Is it better to use a gas or electric heater to back up a solar hot water heater?
We currently have a 1996 vintage gas water heater and are considering replacing it with solar hot water with a 120 gallon electric tank. We have solar pv for electricity. Our 1.8 kW system currently meets our electrical needs.
Gas is generally considered more efficient than electric when heat is what you need.
This is because for every unit of energy that you use in your home, one therm of gas for example, it takes 1/10 of a therm of energy to get the gas to you. For electricity, it takes two times the amount of energy, or 2 kwh to get 1 kwh to your home.
If you can do something else with the hot water like heat your house, then the gas fired back up may make sense. We have an installation of a Phoenix system here in Tucson where the hot water from the water heater heats the house through a hot water coil in the air handler.
Hope this helps.
For more information:
Read Paul Rosen's Q&A "Can you tell me how well solar hot-water heaters work?"