What UEF measures
The Uniform Energy Factor replaced the Energy Factor (EF) in the North American test procedure in 2015. It is the ratio of useful hot-water output to the energy input over a standard test day, so a higher value means less standby and operating loss. A UEF of 0.64 does not mean the heater is 64% efficient at any moment: it is a whole-day test figure that includes standby losses, and a storage tank's daily standing pilot and jacket losses are exactly what drags its UEF below a condensing tankless unit's.
Canada's energy-efficiency regulations for gas-fired storage water heaters set minimum UEF levels by a formula tied to first-hour rating and volume, and NRCan's EnerGuide database publishes the ratings for models sold in Canada. The ENERGY STAR version 5.0 specification for residential water heaters in turn requires gas storage tanks up to 55 gallons to reach UEF 0.81 or better in the medium-draw class, which atmospheric tanks do not meet.
Draw patterns: the comparison trap
UEF is tested under one of four draw patterns chosen by the product class, and the pattern must be stated with the number. A 40-gallon gas tank and a tankless unit are not tested under the same pattern, so their UEF values are not directly comparable. That is why this site states the pattern on every efficiency row and warns against comparing UEF 0.64 (medium draw, RG240T6N) with UEF 0.95 (high draw, RU199iN) as if the arithmetic gap were a savings percentage.
The legacy Energy Factor is a different test again. Rinnai's spec sheet lists EF 0.96 alongside the RU199iN's UEF 0.95; the two must never be mixed in a comparison. Some Canadian models, such as the 60-gallon RG260T6N, are still certified under CSA P.3-04 with an EF rather than a UEF.
What efficiency does not tell you
UEF says nothing about the cost of the fuel, the temperature rise you actually need, your usage pattern, or the quality of the installation. A high-UEF tankless unit that is oversized for the home or served by an undersized gas line performs worse than its rating. WaterHeating.ca publishes no savings claims: the honest statement is that a condensing unit wastes less standby energy than an atmospheric tank, and the size of the saving depends on how the home uses hot water.
Comparing the documented models
Within the same product class and draw pattern, the numbers order themselves: the atmospheric Defender tanks rate 0.54 to 0.64 medium draw; the power-vent RG2PV family reaches 0.72 medium draw because electronic ignition removes the standing pilot; condensing tankless units reach 0.95 at a high-draw pattern. Those are different classes rated under different conditions, and the guide to choosing between them is the tank-versus-tankless comparison, not a subtraction of UEF figures.