We assume a water heater operating on natural gas, fuel oil or propane is 70% efficient, and a water heater operating on electricity is 100% efficient.
For example, if your water heater is fueled by natural gas and your solar water heater produces 100 Therms of energy per year, the natural gas saved would be 100 Therms/0.70, or about 143 Therms of natural gas. This is our estimate for the amount of natural gas you would save on your utility bill in a year.
Please note, the actual efficiency of water heaters run on natural gas, fuel oil, propane, electricity or other fuels varies. We recommend you work with a renewable energy Professional who can better analyze your specific situation.
Go to www.solar-estimate.org