First-hour rating (FHR)
The first-hour rating is how many gallons of hot water a storage water heater can deliver in one hour starting from a full tank — the single most important sizing number for tank-type heaters, more informative than tank size.
FHR combines stored hot water with how fast the unit reheats incoming cold water, so a 50-gallon tank with a strong burner can out-deliver a 60-gallon tank with a weak one. DOE sizing guidance matches FHR to your busiest hour: roughly 45+ gallons for 1–2 people, 55–62 for 3–4, and 75+ for five or more.
Size from your real peak hour — back-to-back showers, laundry, and dishes — not from the number printed on the tank. Two households with the same tank can have completely different experiences if their peak hours differ.
| What it measures | Gallons of hot water delivered in one hour, starting from a fully heated tank |
|---|---|
| Test draw rate | 3.0 ± 0.25 GPM; each draw ends when outlet temperature falls 25 °F below that draw’s peak |
| Rule of thumb | FHR ≈ 70% of tank volume + one hour of recovery |
| Certified heat pump range | 45–134 gallons (median 75) across ENERGY STAR heat pump models |
| Certified gas storage range | 86–206 gallons — high burner input makes recovery, not storage, the dominant term |
| Sizing bands | Under 49 gal → 1–2 people · 49–61 → 2–3 · 62–74 → 3–4 · 75+ → 5 or more |
How the DOE test produces the number
The first-hour-rating test starts with the tank fully heated at a 125 °F thermostat setting and 58 °F inlet water. Water is drawn off at 3.0 GPM until the outlet temperature drops 25 °F below the highest temperature seen during that draw, at which point the draw stops and the heater is left to recover. As soon as the outlet climbs back near its earlier peak, another draw begins. The clock runs for one hour, and a draw that starts inside the hour counts in full.
That structure is why FHR almost always exceeds the tank’s rated volume. A 50-gallon tank does not empty and stop: while you are pulling hot water off the top, the burner or elements are already reheating the cold water arriving at the bottom. The rating is storage plus one hour of recovery, minus the portion of the tank that has cooled past the 25 °F cutoff.
It also explains why the number is honest about blending. Real tanks stratify — hot on top, cold on the bottom, with a transition layer that thickens as you draw. The 25 °F cutoff is a proxy for the moment a shower would start feeling cool, so FHR counts genuinely usable gallons rather than every gallon in the shell.
Working out your own peak hour
DOE’s sizing worksheet assigns gallons to each activity: a shower is 10 gallons, a load of laundry 7, an automatic dishwasher 6, food prep 5, washing hair 4, hand dishwashing 4, and shaving 2. Add up only what happens in your busiest single hour, then pick a heater whose FHR is within a gallon or two of that total. Daily consumption is the wrong metric — 80 gallons spread over a day is trivial, and 60 gallons between 6:30 and 7:30 a.m. is what leaves someone standing in a cold shower.
A worked example: two adults shower back to back (20 gallons), a teenager showers and washes hair (14), someone shaves (2), and the dishwasher runs on the way out (6). That is a 42-gallon peak hour, comfortably inside a 50-gallon electric tank rated around 58–62 gallons FHR. Shift the laundry into that same hour and add a fourth shower and you are at 59 — still fine, but a 40-gallon electric at roughly 50 gallons FHR would not cover it.
Two things bend the arithmetic. Low-flow showerheads at 1.5–1.8 GPM cut a 10-minute shower to about 7–8 gallons of mixed water, of which maybe 5 is hot. And a household that showers in the evening rather than the morning often has a naturally staggered peak, which is why identical families in identical houses report very different experiences with the same tank.
Why two “50-gallon” tanks differ by 40 gallons of FHR
Recovery is the variable. A 4,500-watt electric element puts about 15,400 BTU/h into the water and reheats roughly 20 gallons an hour through a 90 °F rise. A 40,000 BTU/h atmospheric gas burner at 76% recovery efficiency does about 40 gallons an hour; the 65–100 kBTU/h condensing burners in the ENERGY STAR gas storage listings on this site do 80–130. Same tank, wildly different first hours: certified gas storage models here run from 86 to 206 gallons FHR while holding 33 to 112 gallons of water.
Heat pump tanks sit at the other end. In heat-pump-only operation the compressor adds heat at roughly a third the rate of a resistance element, so the certified FHR of a hybrid model leans heavily on its backup elements running in the default mode the test uses. The nominal 50-gallon heat pump models in the certified data land at a median 69 gallons FHR, and nominal 80-gallon models at 91 — the extra storage, not extra recovery, is doing the work.
The practical consequence: never compare a tank by gallons across fuels. A 40-gallon gas heater and a 50-gallon electric can deliver the same first hour, and an 80-gallon heat pump in heat-pump-only mode can under-deliver against a 50-gallon gas tank despite holding 60% more water.
Where the rating stops being useful
FHR is a one-hour figure and says nothing about the second hour. Once storage is spent, you are living on recovery alone — around 20 GPH for electric resistance, 10–15 for a heat pump compressor, 40+ for gas. A household whose demand is a long plateau rather than a spike (a rental with staggered showers from 6 a.m. to 10 a.m.) should weight recovery more heavily and FHR less.
Tankless units have no first-hour rating at all, because they have no storage to draw down. Their equivalent is sustained flow in GPM at your temperature rise, which is a different question with a different answer: a tankless never runs out but caps how many fixtures can run at once, while a tank does the reverse.
Finally, the certified FHR for a hybrid heat pump reflects its as-shipped mode. Switch the unit to heat-pump-only for maximum efficiency and the real first hour drops, sometimes by 15–25 gallons. If you plan to run in that mode, size the tank as though the elements do not exist.
Frequently asked questions
What is a good first hour rating for a family of 4?
Most families of four peak somewhere between 62 and 75 gallons in their busiest hour, so a first-hour rating in that band is the usual target — a 50-gallon gas tank, a 66–80 gallon electric, or a 65–80 gallon heat pump. Add the activities in your own peak hour (10 gallons per shower, 7 per laundry load, 6 per dishwasher cycle) and buy an FHR within a gallon or two of that total rather than trusting the household-size shorthand.
Is first hour rating the same as tank size?
No. Tank size is how much water the heater holds; first-hour rating is how much hot water it can deliver in an hour, which includes everything the burner or elements reheat during that hour. FHR is normally larger than the tank — a 50-gallon gas heater typically rates 80–90 gallons — and two tanks of identical volume can differ by 40 gallons of FHR depending on input rate.
Where do I find the first hour rating of my water heater?
It is printed on the yellow EnergyGuide label as “first hour rating,” listed on the manufacturer’s spec sheet, and carried in the DOE and ENERGY STAR certification records — the figure shown on every storage-tank model page on this site. If a listing quotes only gallons and no FHR, it is withholding the number that actually decides whether the unit fits your household.
What is the difference between first hour rating and recovery rate?
Recovery rate is gallons per hour reheated through a stated temperature rise (usually 90 °F) and describes the heater in steady state. First-hour rating is storage plus roughly one hour of that recovery, measured under the DOE draw test. Recovery rate matters for a long, sustained demand; FHR matters for a morning spike.
Does a tankless water heater have a first hour rating?
No — with no stored water there is no first hour to rate. Size a tankless by the flow it can sustain in GPM at your local temperature rise, which is the maximum number of fixtures that can run simultaneously. Tank buyers size for a spike; tankless buyers size for concurrency.