Water Heater First-Hour Rating, Explained
One number on the yellow label tells you more than the capacity on the badge ever will.
First-hour rating is the gallons of hot water a tank can deliver in one hour starting from full. It combines stored volume with recovery rate, which is why a 50-gallon gas tank can be rated at 91 gallons and a 50-gallon electric one at 63. It is printed on the yellow EnergyGuide label as "Capacity (first hour rating)".
If you take one thing from this site, take this. Tank volume tells you what is in the tank. First-hour rating tells you what you can actually get out of it on the morning everybody showers — and those are different numbers, sometimes by 40 gallons.
What the number actually measures
The test draws hot water out of a full tank and measures how much usable hot water comes out over an hour, while the unit reheats as fast as it can. So the figure depends on three things:
- Stored volume — how much hot water is there at the start.
- Heat source — gas burner or electric element, and how big it is.
- Recovery rate — how fast the unit replaces what you drew, which follows directly from the burner or element size.
That is why first-hour rating is a fair comparison between a gas unit and an electric one, and tank volume is not. It is also why two units in the same family with the same capacity can differ: the A. O. Smith Signature 100 50-gallon unit is rated at 91 gallons in the first hour with a 50,000 BTU burner and 77 gallons with a 40,000 BTU one.
Where to find it
On the yellow EnergyGuide label on the unit, labeled "Capacity (first hour rating)" in the top left. It is also on the manufacturer's specification sheet, and most retailer listings include it — though not all of them, and a listing that omits it is worth distrusting slightly.
Typical first-hour ratings by fuel and size
| Unit | Capacity | Heat source | First-hour rating |
|---|---|---|---|
| Rheem Performance gas | 40 gal | 36,000 BTU | 65 to 68 gal |
| A. O. Smith Signature 100 | 50 gal | 40,000 BTU | 77 gal |
| A. O. Smith Signature 100 | 50 gal | 50,000 BTU | 91 gal |
| Bradford White Defender direct vent | 50 gal | 40,000 BTU | 83 gal |
| Rheem Performance Platinum electric | 50 gal | 4,500 W x 2 | 63 gal |
| Rheem Gladiator electric | 50 gal | 5,500 W x 2 | 63 gal |
| Rheem ProTerra hybrid | 50 gal | Heat pump + 5,000 W | 67 gal |
| A. O. Smith ProLine XE heat pump | 66 gal | Heat pump | 79 gal |
| A. O. Smith Voltex MAX heat pump | 66 gal | Heat pump | 83 gal |
| Rheem Professional Classic electric | 80 gal | 4,500 W x 2 | 78 gal |
Two things in that table are worth sitting with. The Bradford White gets 83 gallons out of a 40,000 BTU burner where the A. O. Smith gets 77 from the same input — design matters, and in this case a sediment-reducing dip tube is part of why. And the 80-gallon electric tank's rating is lower than its own volume, which is what slow recovery looks like on paper.
How to use it to size your tank
- Estimate your peak-hour demand in hot gallons. Roughly 14 hot gallons per 10-minute shower, 4 to 6 for a dishwasher cycle, 40 to 50 mixed gallons for a standard tub.
- Add up the draws that fall inside the same hour on your worst morning.
- Buy a unit whose first-hour rating is at or above that number. Not whose capacity is at or above it.
- Sanity-check the recovery figure separately if your household has a second peak in the evening. A high first-hour rating from a large tank and a weak burner will not refill in time for a 7 pm bath.
Recovery rate is the other half
Recovery rate is gallons per hour at a stated temperature rise, and the rise matters as much as the number. Rheem publishes 21 gallons per hour at a 90°F rise for its Performance Platinum electric tank; a 36,000 BTU gas unit does about 36 gallons an hour. An unqualified recovery figure is as meaningless as an unqualified GPM.
Use first-hour rating to size for the morning rush. Use recovery rate to understand what happens after it. A household with two separate peaks cares about both.
What first-hour rating does not tell you
- Running cost. That is UEF. A 91-gallon first-hour rating on a 0.62 UEF gas tank costs far more to run than a 67-gallon rating on a 3.88 UEF heat pump.
- Simultaneous flow. A tank with a huge first-hour rating still cannot supply two showers at a flow rate the pipework will not carry.
- Anything about tankless units. Tankless has no stored volume, so first-hour rating does not apply. Use GPM at a stated temperature rise instead.
- Longevity. That is the anode rod, the tank lining and the water chemistry. See how long water heaters last.