Energy factor is a measure of efficiency for a variety of appliances, including water heaters. The greater the energy factor, the more efficient the appliance is.
Determining Energy Factor
A water heater’s energy factor (EF) is a measure of useful energy coming out of the water heater, divided by the amount of energy going in. The energy factor is based on the amount of hot water produced per unit of fuel consumed per day.
Electric water heaters typically have energy factors between 0.75 and 0.95. This means that between 75 and 95 percent of the energy used to heat the water ends up being delivered to your taps as usable hot water. The typical energy factor for a gas water heater is between 0.5 and 0.7. Manufacturers are continually changing designs, insulation and other features to make water heaters more efficient.
Energy Factor and Annual Operating Costs
Because of price differences with different fuel sources, a higher energy factor doesn’t always mean a lower annual operating cost. Electric water heaters are more efficient than gas water heaters, largely because gas water heaters lose some of their energy up the vent. But because gas is much less expensive than electricity, gas heaters with a lower EF are actually less expensive to operate annually than electric models. Heat pump electric models, which use energy from air to heat water, have an EF over 2.0 and use one-third to one-half the electricity as standard electric water heaters.
For these reasons, a water heater should not be chosen based only on its energy factor. Fuel type, size, overall cost, and first hour rating should also be considered.