Uniform Energy Factor (UEF)
UEF is the standard efficiency rating for US water heaters: hot-water energy delivered per unit of energy consumed under a standardized DOE test. Higher is better — gas units score around 0.8–0.98, heat pump models 3.0–4.5.
UEF replaced the older Energy Factor (EF) in 2017 and is measured under one of four standardized draw patterns, so two models with the same draw pattern are directly comparable. It rolls standby losses, cycling losses, and burner or compressor efficiency into a single number.
A UEF above 1.0 is impossible for anything that makes heat directly, which is why heat pumps dominate the efficiency rankings: they move heat from surrounding air instead of generating it, delivering 3–4 units of heat per unit of electricity.
| Test procedure | DOE 10 CFR Part 430, Appendix E — mandatory for ratings since June 2017 |
|---|---|
| Test conditions | 58 °F inlet water, 125 °F thermostat setting, 67.5 °F ambient air |
| Draw patterns | Very Small (~10 gal/day), Low (~38), Medium (~55), High (~84) |
| Certified gas storage | UEF 0.86–0.93 (median 0.89) across ENERGY STAR models |
| Certified gas tankless | UEF 0.95–0.98 — effectively all condensing designs |
| Certified heat pump | UEF 2.52–4.5 (median 3.7); 120 V plug-in models 2.8–3.5 |
What the test actually does
The DOE procedure runs a water heater through a scripted 24-hour day: a fixed schedule of draws at a fixed flow, with 58 °F water entering, a 125 °F thermostat setting, and 67.5 °F room air. Energy going in is metered; hot-water energy coming out is calculated from flow and temperature. UEF is the second divided by the first.
Because the test spans a full day rather than a single burn, it captures three separate losses in one number. Combustion or compressor efficiency is the obvious one. Standby loss — heat leaking through the tank wall between draws — is the reason a heavily insulated tank beats a thin one with the same burner. Cycling loss covers the energy wasted every time a burner lights, warms its heat exchanger, and shuts down again.
That is the whole point of the metric. Spec-sheet thermal efficiency describes the burner at its best minute; UEF describes the appliance over a realistic day, which is what shows up on the bill. It is also why a tankless unit, with no tank to lose heat from, posts a higher UEF than a storage model using a similar burner.
Why heat pumps score above 1.0
Anything that turns fuel or current directly into heat is capped at 1.0 by conservation of energy — a resistance element is nearly perfect at 0.92–0.95 UEF once tank losses are counted, and a burner loses whatever goes up the flue. A heat pump does not make heat; it moves it, running a refrigeration cycle in reverse to pull energy out of room air and dump it into the tank. Spending 1 kWh to relocate 3.7 kWh of ambient heat is not a violation of anything, and it is why the certified ceiling on this site sits at UEF 4.5.
The catch is that the borrowed heat has to come from somewhere. In a 70 °F garage there is plenty; in a 45 °F basement in February the compressor works harder and the real-world coefficient of performance falls well below the 67.5 °F test condition. In conditioned space the heat is partly stolen from your furnace, so the net saving is smaller than the rating implies — the reason the standard advice is to put a heat pump water heater in a garage, utility room, or unconditioned basement.
UEF also cannot see your operating mode. A hybrid unit is tested as shipped, elements and all; run it in heat-pump-only mode and efficiency rises above the label, run it in electric mode all winter and it collapses toward 0.95.
Turning UEF into an annual bill
A typical household needs roughly 3,800 kWh of delivered hot-water energy per year. Divide by UEF to get purchased energy, then multiply by your rate. A UEF 0.90 gas tank buys about 4,200 kWh-equivalent, or 144 therms — around $209 a year at $1.45 per therm. A UEF 3.9 heat pump buys about 975 kWh, or $166 at $0.17 per kWh. A plain electric resistance tank at UEF 0.92 buys 4,130 kWh: about $700.
The gas-versus-heat-pump gap is far narrower than the 0.90-versus-3.9 ratio suggests, because a therm of gas costs roughly a third of what the same energy costs as electricity. That single fact decides most upgrade decisions: heat pumps crush electric resistance everywhere, and beat gas comfortably only where electricity is cheap, gas is expensive, or a time-of-use rate lets the tank heat off-peak.
The yellow EnergyGuide label does this arithmetic for you, but with a national average rate that is usually a year or more stale and never matches your utility. Take the label’s estimated annual therms or kWh — not its dollar figure — and multiply by the rate on your own bill.
Three ways UEF comparisons go wrong
Comparing across draw patterns is the most common error. A heater tested on the High pattern spreads its standby losses over 84 gallons a day; the same design tested on Low spreads them over 38 and scores lower. Only models sharing a draw pattern are directly comparable, which is why every listing on this site shows the pattern next to the rating.
The second is treating pre-2017 Energy Factor numbers as interchangeable. EF used a different draw schedule and different inlet conditions, so a 0.95 EF tankless and a 0.95 UEF tankless are not the same appliance. Any comparison that mixes the two is marketing, not measurement.
The third is buying efficiency instead of capacity. UEF says nothing about how much hot water arrives at once — that is first-hour rating for tanks and sustained GPM for tankless. An undersized UEF 4.2 heat pump will produce cold showers very efficiently. Size first, then optimise the rating within the sizes that fit.
Frequently asked questions
What is UEF on a water heater?
Uniform Energy Factor is the ratio of useful hot-water energy delivered to total energy consumed over a standardized 24-hour DOE test day, including standby and cycling losses. It has been the required US efficiency rating since June 2017 and appears on the EnergyGuide label, the ENERGY STAR certification record, and every model page on this site.
What is a good UEF rating?
It depends on the technology, because the scales do not overlap. For gas storage, 0.86–0.93 is the certified high-efficiency band against federal minimums near 0.60–0.64. For gas tankless, 0.95 and up. For heat pumps, 3.5 or better is strong and the market tops out around 4.5. A plain electric resistance tank sits at roughly 0.92 no matter what it costs.
What is the difference between UEF and EF?
EF was the pre-2017 rating and used a single test regime for all sizes. UEF replaced it with four draw patterns matched to unit capacity, revised inlet and ambient conditions, and a more realistic draw schedule. The numbers are not convertible: a model’s UEF is often a few points below its old EF, and comparing one against the other tells you nothing.
Is a higher UEF always cheaper to run?
Only within the same fuel. Across fuels, price per unit of energy dominates: at national average rates a UEF 0.90 gas tank costs about $209 a year while a UEF 3.9 heat pump costs about $166 — a 4× efficiency advantage worth roughly 20% in dollars, because electricity costs about three times as much per delivered unit as gas. Run the numbers at your own rates before assuming the higher UEF wins.
Can UEF be higher than 1.0?
Yes, but only for heat pumps. A UEF above 1.0 means the appliance delivered more heat energy than it consumed, which is possible only when it is moving existing heat rather than creating it. Anything that burns fuel or runs current through an element is capped below 1.0.