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Calculating Annual Operating Cost
To estimate the annual operating cost of a solar water heating system, you need the following:
 The system's solar energy factor (SEF)
 The auxiliary tank fuel type (gas or electric) and costs (your local utility can provide current rates).
Then, use the following calculations.
With a gas auxiliary tank system:
You need to know the unit cost of fuel by Btu (British thermal unit) or therm. (1 therm = 100,000 Btu)
365 × 41,045/SEF × Fuel Cost (Btu) = estimated annual cost of operation
OR
365 × 0.4105/SEF × Fuel Cost (therm) = estimated annual operating cost
Example: Assuming the SEF is 1.1 and the gas costs $1.10/therm
365 × 0.4105/1.1 × $1.10 = $149.83
With an electric auxiliary tank system:
You need to know or convert the unit cost of electricity by kilowatthour (kWh).
365 × 12.03/SEF × Electricity Cost (kWh)= estimated annual operating cost
Example: Assuming the SEF is 2.0 and the electricity costs $0.08/kWh
365 X 12.03/2.0 X $0.08 = $175.64
Source: US department of Energy (DOE)
