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.