UEF explained: what the efficiency number actually means in dollars
Updated · Reviewed against the current HotWaterAtlas dataset
Short answer: UEF is "useful hot-water energy out ÷ energy in" over a standardized test day. A gas tank at UEF 0.90 wastes a tenth of every therm; a heat pump at UEF 4.5 delivers 4.5× its electricity as heat. Divide your annual water-heating energy by UEF and multiply by your rate — that's the whole trick.
How the number is measured
The DOE test runs each unit through a fixed 24-hour schedule of hot-water draws — its draw pattern — while metering energy in and hot-water energy out. Standby losses, cycling losses, and burner or compressor efficiency all land in one number. That's what makes UEF comparable in a way spec-sheet burner efficiency never was: it's the whole day, not the best minute.
Where the certified market sits
| Category | UEF range (certified) | What it means physically |
|---|---|---|
| Gas storage | 0.86–0.93 | Condensing burners, insulated tanks |
| Gas tankless | 0.95–0.98 | Condensing, no standby tank losses |
| Heat pump | 2.52–4.5 | Moves heat instead of making it |
Anything that burns fuel or runs current through a resistor is capped at UEF 1.0 by thermodynamics. The gap from ~0.95 to 4.5 is not incremental engineering — it's a different physical mechanism, which is why the UEF 4.0+ list is entirely heat pumps. The current certified ceiling is the GE Profile Heat Pump 50-Gallon (PH50S10BNY01) at UEF 4.5 (about $187/year to run at national average rates).
Turning UEF into dollars
A typical household's hot water is roughly 10–11 kWh of delivered heat per day (≈ 3,800 kWh/year). Divide by UEF for purchased energy, then multiply by your rate:
- UEF 0.90 gas tank: 3,800 ÷ 0.90 ≈ 4,200 kWh-equivalent ≈ 144 therms ≈ $210/yr at $1.45/therm.
- UEF 3.9 heat pump: 3,800 ÷ 3.9 ≈ 975 kWh ≈ $165/yr at $0.17/kWh — and far less on cheap or off-peak power.
Every model page on this site does this conversion for you from the certified annual energy use — the assumptions are documented in the methodology. For the full market comparison, see the operating-cost guide.
Three caveats when comparing UEF
- Compare within a draw pattern. UEF is only apples-to-apples between models tested on the same usage pattern — our comparison pages show the pattern alongside the rating.
- UEF ≠ capacity. A high-UEF unit that's too small still runs cold. Size by first-hour rating or GPM first, then optimize UEF.
- Old EF ratings aren't comparable. The pre-2017 EF test used a different procedure; treat EF-to-UEF comparisons as marketing, not measurement.