Uniform energy factor and the yellow label

Uniform energy factor is the efficiency metric the U.S. Department of Energy uses for residential water heaters under the current federal test. A higher number means the heater delivered more hot water per unit of fuel in that test. The number lives on the yellow EnergyGuide label and in the specification sheet.

It replaced the older energy factor for the current test procedure. An EF from a 2012 brochure and a UEF from a 2026 label are not the same measurement, and ranking them as if they were is a good way to buy the wrong heater. The test method sits in 10 CFR Part 430.

The bins and the minimums for a voluntary ENERGY STAR mark sit on ENERGY STAR’s residential water-heater criteria page, which is updated when the thresholds change. This page explains how to read the number. It does not freeze a threshold that the criteria page can revise.

What the test is doing

The older energy-factor test used one daily draw pattern for a wide range of heaters. The UEF test sorts heaters into bins and gives each bin its own draw pattern, so a small heater is not scored on a large household’s day and a large heater is not scored on a tiny one. For storage heaters the bins are based on first-hour rating.

The boundaries widely published with the DOE procedure are: very small, under 18 gallons first-hour rating; low, from 18 up to but not including 51; medium, from 51 up to but not including 75; and high, 75 and above. For tankless heaters the bins use maximum gallons per minute instead: very small under 1.7, low from 1.7 up to but not including 2.8, medium from 2.8 up to but not including 4, and high at 4 and above.

Confirm the current edges in the Code of Federal Regulations before you rely on a boundary in a dispute. A heater that sits one gallon from a bin edge can move categories if the rule is amended or if a different first-hour rating is certified.

Inside a bin, a higher UEF is more efficient on the test. Across bins, you are looking at heaters that were scored on different days. A medium-bin gas heater and a high-bin gas heater are not a pure efficiency contest.

Pick the bin by sizing the peak hour first, using how to size a tank or how to size a tankless heater. Then compare UEF among heaters that can actually do that hour or that flow. Buying the highest UEF in the store, in a bin too small for the household, saves energy by delivering disappointment.

The test conditions are not your Tuesday morning. Inlet temperature, set point, and draw pattern are standardized so two factories can be compared. Your groundwater and your showers will not match the lab.

UEF is a comparison tool. It is not a prediction of your bill. The bill prediction uses the label’s estimated energy use and your actual fuel price.

The yellow label

Federal Trade Commission EnergyGuide labels on covered water heaters show the estimated annual energy use and an estimated annual cost. The cost is computed with a national average rate that the label prints.

If your electricity is much more expensive than that average, or your gas is much cheaper, the dollar ranking on two labels can reverse. Multiply the therms or the kilowatt-hours by your own marginal rate.

What heaters cost makes the same point for operating cost. The first-hour rating on a storage label is the sizing number. Do not skip it to stare at the dollar.

Heat pump water heaters can show a UEF well above 1, often in the 3s, because they move heat rather than creating all of it from the electricity they consume. A resistance electric tank sits near 0.9: almost all of the electricity becomes heat in the water, and the rest is standby loss.

A UEF of 3.5 against a UEF of 0.92 is a real and large difference in electricity use for the same test bin. It is the strongest argument for a heat pump when the room can host one, as the heat pump page conditions it. It is not an argument that the heat pump costs less to install. Installation is a separate line.

Gas heaters show UEF numbers that look “worse” than electric resistance, often from the 0.60s for older atmospheric designs into the 0.80s and 0.90s for condensing designs. The gas number is lower because combustion sends heat out the flue and because the fuel is being measured at the burner, not because the gas heater is a defective electric heater.

You cannot subtract a gas UEF from an electric UEF and call the difference savings.

Convert both to your fuel bill, in dollars per year, using the energy use on the label and your rates. Gas or electric walks that comparison.

Condensing gas heaters earn their higher UEF by capturing flue-gas heat. The vent required to do it is the condensing page, and it may cost more than the UEF gain is worth if the old vent was sound and the household’s hot-water use is small.

ENERGY STAR is a threshold, not the UEF itself

ENERGY STAR sets minimum criteria by product class. A heater can have a respectable UEF and still miss the current ENERGY STAR cut for its class. A heater can meet the cut and still be the wrong size. Use the ENERGY STAR mark as a screen if you want a unit that clears a public bar, then use the criteria page to see what that bar is this year.

Do not pay for the mark if the installation cannot meet the manufacturer’s clearance, air-volume, or vent rules. An efficient heater installed so that it short-cycles, or so that a heat pump stays in electric mode, will not produce the test number in your closet.

The federal tax credit that referred to high-efficiency heat pump water heaters had its own eligibility, tied to efficiency tiers and to dates. It is not the same thing as the ENERGY STAR mark, and for property placed in service after December 31, 2025 the IRS says that 25C credit is not allowed. Do not treat “high UEF” as a synonym for “there is a federal credit.” The cost page cites the IRS documents.

What UEF leaves out

UEF does not measure how well the installer sized the vent or the gas line. It does not measure your groundwater.

It does not measure scale. A tankless unit with a high UEF, full of scale, will not deliver the charted flow. A storage heater with a high UEF and a broken dip tube will not deliver its first hour.

Maintenance and diagnosis still exist: hard water, runs out of hot water. UEF does not measure standby loss in your particular uninsulated garage beyond what the test’s ambient conditions assumed. It does not measure the cost of a service upgrade.

It is silent on scalding. A higher set point can change delivered energy and can injure people. The set point is a safety choice, covered in temperature and scalding, not an efficiency trick.

Checklist: use the number without fooling yourself

Size the peak hour or the tankless flow first. Bin follows size.
Compare UEF inside the same fuel and the same bin.
Do not compare an old energy factor to a new UEF as equals.
Read estimated energy use, then apply your own rate. Ignore the label’s dollar if your rate is far from the printed average.
Treat a heat pump UEF above 1 as a real electric saving, conditional on the room and the mode.
Do not treat a higher gas UEF as “worse” than an electric 0.92. Convert both to money.
Check ENERGY STAR criteria separately, on the current page.
Do not assume a federal credit from a high number. Check the date and the IRS rule.

Sources

U.S. Department of Energy, uniform energy factor test procedure, 10 CFR Part 430 (ecfr.gov). ENERGY STAR, residential water heater key product criteria (energystar.gov). U.S. Department of Energy, Energy Saver, “Sizing a New Water Heater,” for first-hour rating as the capacity figure on the label and for the instruction not to size by tank gallons alone.

U.S. Department of Energy, Energy Saver, “Which Water Heater Is Right for You?,” for the heat-pump efficiency multiple that a UEF above 1 is expressing. Federal Trade Commission EnergyGuide program, for the annual-cost figure’s dependence on a published average fuel price. Internal Revenue Service, Energy Efficient Home Improvement Credit, for the credit window ending December 31, 2025.