UEF explained: why heat pumps rate 2.52 to 4.50 and gas units 0.86 to 0.98

The Uniform Energy Factor (UEF) is the hot-water energy a heater delivers divided by the energy it uses over the DOE 24-hour simulated-use test. On the certified list, heat-pump sets rate from 2.52 to 4.50 and gas units from 0.86 to 0.98. The two ranges never overlap, and a UEF only compares with another UEF from the same draw pattern.

Why the scales are so far apart

A gas water heater turns fuel into heat, and some of that heat leaves through the flue and the tank walls, so its UEF stays below 1. A heat pump moves heat from the surrounding air into the water and uses electricity mainly to run the compressor, so it can deliver more heat energy than the electricity it draws. That is how a UEF above 1 is possible.

Because the two scales measure different energy inputs, this site never states one as a multiple of the other. A heat-pump UEF and a gas UEF sit on separate rows of every table, and the Heater finder does not subtract one from the other.

The range inside each draw pattern

The test uses one of four daily water-use profiles, chosen from the heater’s first-hour rating or flow. The table below keeps each profile on its own row and counts each rating set once.

Three things stand out. The heat pumps spread widely: 2.85 to 4.50 at the high pattern. The gas tankless units are packed tightly: of the 39 high-draw tankless sets, 19 rate 0.95, 13 rate 0.96, 1 set rates 0.97 and 6 rate 0.98. And the gas storage tanks, all tested at the high pattern, run from 0.86 to 0.93.

UEF and annual energy move together

Inside one draw pattern, the UEF and the rated annual energy are two views of the same test. Across the 58 high-draw heat-pump sets the correlation between them is −0.83: as UEF rises, rated kWh falls. Friedrich PROH65 T2 FD310BM has the lowest rated use of the group, 900 kWh/yr at a UEF of 3.50; Smart Solar SSG2-ES 120 has the highest, 1,741 kWh/yr at 2.85.

The relation is not exact, because rated volume and first-hour output also shape the test result. Two sets with the same UEF can differ by tens of kWh a year.

The same UEF in two draw patterns

Here is why the draw pattern matters. A. O. Smith HPTV-80 2, tested at the high pattern, and Rheem PROPH50 T0 RH120-M, tested at the medium pattern, have the same UEF, 3.00. Their rated use is 1,654 kWh/yr and 1,072 kWh/yr: the high pattern draws more water each day, so the same efficiency produces a larger number. Comparing a UEF, or an annual energy figure, across patterns mixes two different test days.

What the 24-hour test counts

The simulated-use test runs a full day of hot-water draws set by the draw pattern, then counts all the energy the heater used over that day. That includes the energy spent keeping stored water hot between draws, so a tank that loses little heat through its walls scores better than one that loses more, even with the same burner or compressor.

The heat-pump spread by subtype

The wide heat-pump range is partly a matter of build. At the high pattern, the median 240-volt integrated set rates 4.00, the median split system 3.77 and the median 120-volt plug-in set 3.40. The subtype comparison takes this further.

Among the gas tanks, the 4 standard storage sets rate 0.88 to 0.90, and the 15 residential-duty commercial sets 0.86 to 0.93. The highest-rated tank, at 0.93, still sits below the weakest tankless set.

Solar is a different factor again

Solar systems carry a solar uniform energy factor (SUEF) from their SRCC OG-300 rating. The SUEF counts the sun’s contribution as free and only the backup energy as used, so values far above the heat-pump range are possible. The site lists these values with their source but does not rank them; see the solar page.

How to use UEF on this site

Every UEF rank on this site names its draw pattern and its type, for example “2nd highest of 38 medium-draw heat-pump sets”. In the Heater finder, the comparison table replaces the UEF difference with “not comparable” when two sets come from different patterns. The figures describe the test, not the bill for any household; the site does not convert them into money.