Solar Energy Factor (SEF)
The SEF is the energy delivered by the system divided by the conventional energy input (energy used by the backup heater plus any energy used to power pumps or controllers in the solar water heating system). The values range from 1 to 11, with most residential systems having a water heater efficiency of 2 or 3. The higher the value, the more energy efficient the system. That means the solar thermal system contributes more to water heating so less conventional energy is required.