Recovery rate is how many gallons an hour a heater can raise through a given temperature rise. It is half of what determines a water heater's real capacity.
The arithmetic
GPH = BTU/h input × efficiency ÷ (8.33 × temperature rise)
The 8.33 is the weight of a gallon of water in pounds, and one BTU raises one pound by one degree.
For electric, convert watts to BTU/h by multiplying by 3.412.
What that gives in practice
A 40,000 BTU gas burner at around 75% efficiency, through a 90°F rise: roughly 40 gallons per hour.
A 4,500 watt electric element through the same rise: roughly 20 gallons per hour.
A 5,500 watt element: roughly 25.
A heat pump water heater in heat pump mode: slower still, which is why they are usually sized a tank size larger.
Gas recovers about twice as fast as electric at typical residential ratings, which is the single most useful fact in comparing the two.
Why electric heaters have two elements and use one
A residential electric water heater has an upper and a lower element, wired so that only one runs at a time.
The upper element has priority: when the top of the tank cools, it heats that first so there is some hot water quickly. When the top is satisfied, control passes to the lower element.
That is why a 4,500 watt heater does not recover at 9,000 watts, and why running two elements simultaneously would need a much larger circuit than houses are wired for.
Temperature rise changes everything
Recovery is quoted at a specific rise, usually 90°F. At a smaller rise the same heater recovers faster; at a larger one, slower.
Incoming water is colder in winter, so the rise is larger, so recovery is slower, so the household runs out of hot water sooner. That is the entire explanation for a complaint that appears every January.
The magnitude is not small: incoming water at 40°F rather than 70°F is a 33% larger rise, and recovery falls by about the same proportion.
What raises recovery
A larger burner or element, within what the tank and the circuit allow. Higher-output residential gas heaters exist and they recover proportionally faster.
Higher storage temperature with a mixing valve, which does not change the recovery rate and does increase the usable stored volume, which has a similar effect on the household's experience.
Gas instead of electric, where a gas connection exists.
What lowers it
Sediment on the bottom of a gas tank, which insulates the water from the burner. Flushing restores it.
Scale on electric elements, which does the same thing.
A failed element or thermostat, where a two-element electric heater running on one gives roughly half the recovery and about half the hot water. This is a common and easily missed fault, and the symptom is exactly "we started running out of hot water".
Recovery against first hour rating
First hour rating combines the stored volume with an hour of recovery, and it is the number to size against.
Recovery on its own tells you what happens after that first hour — in a household with a long peak, or several peaks, recovery matters more than the tank.
Both are on the EnergyGuide label, and the second explains the first.
Testing recovery at home
Run the hot water until it goes cold, note the time, and then check how long until it is hot again at a tap.
Compare that against the published recovery rate and the tank size. A heater taking substantially longer than it should has a fault — sediment, scale, a dead element, or a thermostat out of calibration — and all four are worth finding before concluding the heater is too small.
Sizing around slow recovery
Where recovery is inherently slow — electric, or a heat pump unit — the answer is stored volume.
A larger tank at the same recovery rate delivers a bigger peak, which is exactly why electric heaters are commonly a size larger than gas ones for the same household. Buying an undersized electric tank and expecting the recovery to cover it is the mistake that produces most complaints.