What a first-hour rating tells you, and what rated volume doesn’t

The first-hour rating is the number of gallons of hot water a storage water heater delivers in one hour of the DOE test, starting from a full, heated tank. It is not the tank size. Across the 120 storage rating sets on the certified list, the median heat-pump set delivers 1.25 gallons in the first hour per gallon of rated volume, and the median residential-duty commercial gas tank 2.65. Two tanks of the same rated volume can be far apart.

The rating comes from a fixed procedure in the federal test method (10 CFR Part 430, Appendix E). The tank starts full and at temperature, hot water is drawn off, and the heater recovers while the hour runs. Because the clock runs for a full hour, the result includes water the heater reheats during that hour, not only what was stored at the start. That is why a heater with a strong burner can deliver more than its own volume, and a heater that reheats slowly can deliver less.

Same tank size, different first hour

Take the rating sets with a rated volume between 45 and 50 gallons. There are 27 heat-pump sets in that band, and their first-hour ratings run from 50 gal to 77 gal. The 2 standard gas storage sets in the same band rate 95 gal and 118 gal. The 6 residential-duty commercial gas sets rate from 100 gal to 167 gal.

So the label on the tank says little about how much hot water arrives in a busy hour. Rated volume is itself a test figure: a heat pump sold as a 50-gallon model usually rates 45 to 47 gallons. Neither number is wrong; they answer different questions.

Gallons per gallon: a simple way to read the list

Dividing the first-hour rating by the rated volume gives one number per rating set that is easy to compare across sizes. We call it first-hour output per rated gallon. It is our own index, not a DOE or ENERGY STAR figure, and the Test and data page defines it. The medians below count each rating set once, so a design sold under eight brand names does not weigh eight times.

The three heat-pump types sit close together, between 1.16 and 1.27, with the 120 V plug-in units lowest. The gas tanks start where the heat pumps stop: the 4 standard gas storage sets have a median of 2.23.

The spread at the edges is wide. The highest ratio on the list belongs to American PG10 34 100 2V 2, a residential-duty commercial tank with a rated volume of 33 gal, a gas input of 100,000 Btu/h and a first-hour rating of 170 gal: 5.15 gallons per rated gallon. The lowest is Smart Solar SSG2-ES 80, a split-system heat pump that rates 55 gal from 80 gal. Among the 101 heat-pump sets, 7 have a first-hour rating below their rated volume; no gas tank on the list does.

Where the extra gallons come from

The clearest case on the list is a pair of Rheem tanks that differ in one value. Rheem ECE G50-76U CN HE EC3 and Rheem ECE G50-100U CN HE EC3 both rate 50 gal. The first has a gas input of 76,000 Btu/h and a first-hour rating of 136 gal; the second has 100,000 Btu/h and 167 gal. Same rated volume, 31 more gallons in the hour, from the larger burner.

The heat-pump sets stay much closer to their stored volume. The largest heat-pump figure on the list, 134 gal, comes from SanCO2 GS5-45HPC HP + ECO-119GLBK, which also has one of the largest rated volumes, 119 gal. The median heat-pump set rates 77 gal.

The first-hour rating also sets the draw pattern

The test method sorts each storage heater into a draw pattern by its first-hour rating, and the UEF is then measured with that pattern’s daily water use. A heater that rates 75 gallons or more is tested at the high pattern; one that rates 74 is tested at the medium pattern. The draw-pattern explainer shows what that boundary does to the numbers.

Reading the number on this site

Every type and maker page shows rated volume and first-hour rating side by side, and the Heater finder lets you set a minimum first-hour rating for heat-pump and gas storage sets. Tankless units have no first-hour rating; they are rated by maximum flow at a 67 °F rise instead, and the two figures are never compared here. The values are certified test results, not a measure of any household’s use.