The Energy Factor (EF) rating represents the efficiency of the water heater. The higher the EF, the more efficient the water heater unit. Hubbell Model E water heaters are classified by the Department of Energy as residential water heaters and are rated by an Energy Factor (EF) ranging from 0.80 to 0.92. The higher the EF, the more the heater transfers energy to the water using less energy. Please note that the use of R values as a measure of efficiency is incomplete and does therefore does not accurately capture a water heater’s true efficiency as does the EF rating. For additional information check out the Air-Conditioning, Heating, and Refrigeration Institute (AHRI) website at AHRI as well as the American Council for an Energy Efficient Economy (ACEEE) website at ACEEE