Energy Factor (EF) is a rating that measures how efficiently an appliance converts fuel or electricity into useful work, most commonly hot water. The higher the number, the more efficient the appliance. You’ll encounter this term most often when shopping for water heaters, though versions of it also apply to dishwashers and clothes washers.
How Energy Factor Works
For water heaters, Energy Factor represents the amount of hot water produced per unit of fuel consumed over a typical day. Think of it as a ratio: how much useful heat you get out versus how much energy you put in. A water heater with an EF of 0.90 converts 90% of its fuel into hot water, while the remaining 10% is lost.
That lost energy comes from two main sources. The first is heat that escapes through the walls of the tank while the water sits waiting to be used, called standby loss. The second is inefficiency in the burner or heating element itself during the heating cycle. Both of these chip away at overall efficiency, and the Energy Factor captures their combined effect in a single number.
For dishwashers, the calculation works differently. EF is expressed in cycles per kilowatt-hour, essentially measuring how many wash cycles the machine can complete on a set amount of energy. It accounts for both the electricity the machine uses and the energy needed to heat the water during the cycle.
Typical Ratings by Water Heater Type
Energy Factor values vary dramatically depending on the type of water heater, because different technologies waste energy in different ways.
Conventional gas storage water heaters typically fall in the range of 0.50 to 0.70. They lose a significant amount of heat through the flue and through the tank walls. Electric resistance water heaters do better, generally landing between 0.90 and 0.95, because they don’t vent combustion gases and convert electricity to heat more directly.
Heat pump water heaters blow past both of those categories. Instead of generating heat directly, they pull warmth from the surrounding air and concentrate it into the water, a process that can deliver more heat energy than the electrical energy it consumes. ENERGY STAR certified heat pump models now require a rating of 2.20 or higher, with top integrated models reaching 3.30 or above. That means they produce more than three times as much heat energy as the electricity they use.
The Shift to Uniform Energy Factor
If you’re shopping for a water heater today, you’ll likely see UEF (Uniform Energy Factor) on the label rather than the older EF rating. The U.S. Department of Energy introduced UEF to make comparisons between models more accurate and realistic.
The key difference is that UEF assigns water heaters into one of four usage categories, called bins, based on how much hot water a household actually draws. A small tank serving a single person gets tested under different conditions than a large tank serving a family of five. Under the old EF system, all water heaters were tested the same way regardless of size or intended use, which made apples-to-apples comparisons misleading.
Because the testing methods changed, EF and UEF numbers aren’t directly interchangeable. A water heater’s UEF rating will be slightly different from what its EF rating would have been, so when comparing models, make sure you’re looking at the same metric for both.
ENERGY STAR Minimums to Know
ENERGY STAR certification sets minimum UEF thresholds that can serve as a useful benchmark when you’re shopping. For gas water heaters, the minimum UEF is 0.81 for tanks between 20 and 55 gallons, and 0.86 for tanks larger than 55 gallons. Electric resistance models need a UEF of at least 0.95. Gas tankless (on-demand) water heaters require a UEF of 0.86 or higher.
If a model meets these thresholds, it generally qualifies for rebates and tax incentives, which can offset the higher upfront cost of more efficient units.
Energy Factor for Washers and Dishwashers
Water heaters aren’t the only appliances rated this way. Clothes washers use a version called the Integrated Modified Energy Factor (IMEF), which accounts for the electricity the washer uses during the cycle and on standby, the energy needed to heat wash water, and even the energy your dryer will use afterward (since a washer that spins out more water means less drying time). A higher IMEF means a more efficient machine overall.
Dishwashers use the original Energy Factor metric, measured in cycles per kilowatt-hour. The formula divides one by the total energy the machine consumes per cycle, combining both the electricity it draws and the energy needed to heat the water. A dishwasher with a higher EF completes more cycles on the same amount of energy.
What the Numbers Mean in Practice
Energy Factor is most useful as a comparison tool. When you’re choosing between two water heaters of the same type and size, the one with the higher UEF will cost less to operate over its lifetime. The difference can be substantial: upgrading from a standard gas water heater with a UEF around 0.60 to a heat pump model rated at 3.30 can cut water heating costs by 70% or more, and water heating typically accounts for about 15 to 20% of a home’s total energy bill.
Keep in mind that Energy Factor measures efficiency under standardized lab conditions. Your actual savings will depend on how much hot water you use, your local fuel costs, the temperature of your incoming water supply, and where the heater is installed. A water heater in an unheated garage loses more standby heat than one in a warm utility closet, regardless of what the label says.