What does UEF mean on a water heater?
Uniform energy factor is the efficiency rating on every residential water heater. How to read it, and what the ENERGY STAR levels are for each type.
Published 2 min read
In short
UEF, the uniform energy factor, compares how much of the energy a water heater uses ends up as hot water. The higher the number, the less energy for the same hot water.
Ratings span a wide range: about 0.95 for a standard electric tank, 0.81 to 0.95 for ENERGY STAR gas models, and 3.30 or more for an ENERGY STAR heat pump water heater, which moves heat from the air rather than making it.
DOE’s Federal Energy Management Program defines the uniform energy factor as “a metric used to compare the energy conversion efficiency of residential appliances and equipment”. For a water heater, it is the ratio of the energy delivered as hot water to the energy the water heater uses, measured under a standard test. A UEF of 0.95 means 95 units of hot water energy for every 100 units of electricity or gas.
Why can a UEF be above 1?
A heat pump water heater does not make heat from electricity; it moves heat from the surrounding air into the tank, so it can deliver more heat than the electricity it uses. That is why heat pump models are rated above 3 while electric resistance and gas models stay below 1. It is the same idea as a heat pump’s coefficient of performance.
What UEF should I look for?
ENERGY STAR sets minimums by type:
| Water heater type | ENERGY STAR minimum UEF | Other ENERGY STAR requirements |
|---|---|---|
| Heat pump, integrated | 3.30 | First-hour rating of at least 45 gallons per hour; at least 6-year warranty on the sealed system |
| Heat pump, integrated, 120-volt / 15-amp circuit | 2.20 | Same |
| Heat pump, split system | 2.20 | Same |
| Gas storage, over 55 gallons | 0.86 | — |
| Gas storage, 20 to 55 gallons, medium draw | 0.81 | — |
| Gas tankless | 0.95 | Maximum flow of at least 2.8 gallons per minute over a 67 °F rise |
For comparison, DOE’s FEMP lists a less efficient standard electric tank at UEF 0.95 and the best available heat pump model in its comparison at 3.93.
How do I compare two water heaters?
- Same fuel: energy use scales with 1 ÷ UEF. A heat pump water heater at 3.30 uses about 29% of the electricity of a 0.95 resistance tank for the same hot water.
- Different fuels: divide each energy price, in dollars per million Btu, by the UEF. Our heat pump water heater explainer runs that comparison at 2026 average prices.
- Capacity as well as efficiency: UEF says nothing about how much hot water a unit can supply in a busy hour. For storage models, look at the first-hour rating; for tankless models, the flow rate at your temperature rise. ENERGY STAR suggests starting from the gallon capacity of your current storage water heater.