Electric storage water heaters are very simple appliances. Modern units consist of a glass-lined steel tank, surrounded with foam insulation. If you have an old electric water heater, check the type and thickness of insulation. You can see the insulation at one of the holes in the water heater’s outer shell, or around the opening to the combustion chamber. If it has one or two inches fiberglass, the water heater was manufactured before 1991 and will be quite inefficient due to heat loss through the tank’s shell.
The efficiency of water heaters is rated by their Energy Factor (EF). This is the percentage of energy that remains in the hot water you use compared to the energy that was used to heat that water. For example, the best new electric water heaters have an Energy Factor of 0.95. This means that 95% of the energy used to heat the water is still there when it enters your water lines, and that 5% escaped through the walls of the tank before you used the water. Gas water heaters have a much lower Energy Factor than electric units because they also lose energy up the chimney.
Before 1991, fiberglass-insulated electric water heaters had Energy Factors ranging from 0.80 to 0.86. The National Appliance Energy Conservation Act (NAECA) of 1987 required electric water heaters manufactured after 1991 to meet a minimum Energy Factor of 0.88. As a result, manufacturers switched to foam insulation that increased the thermal resistance to as high as R-16 when using 2.5 inches of foam. The best new electric water heaters now have 3 inches of foam insulation.
When you next shop for an electric water heater, remember that electricity is an expensive and precious resource. Choose the best electric water heater available with an Energy Factor of 0.92 to 0.95. If your existing electric water heater has an Energy Factor of 0.86 and the new one is 0.95, you’ll save over 400 kilowatt-hours (kWh) per year or $25-35 per year.
By the way, if you have an old water heater, it may be better to replace it now rather than to wait until your old one fails. If you wait until you have an emergency, as when your water heater springs a leak, your local appliance dealer may not have the most efficient models in stock.
Tank Volume Recommended Best Available
Energy Factor Annual Energy Use (kWh/year) Energy Factor Annual Energy Use (kWh/year)
Less than 60 gallons 0.93 4,721 0.95 4,622
More than 60 gallons 0.91 4,825 0.92 4,773
U. S. Department of Energy, Energy Efficiency and Renewable Energy Division
The Homeowners Handbook to Energy Efficiency contains simple, practical energy-efficiency ideas.