Water Heater Recovery Rate
Calculate how fast a water heater reheats — the recovery rate in gallons per hour — from its energy input, efficiency and the temperature rise.
Calculator
At 80% efficiency and a 70 °F rise, this heater recovers about 54.8 gallons per hour. Recovery (how fast a tank reheats) matters most for back-to-back demand; a bigger burner or higher efficiency recovers faster.
Recovery rate is how many gallons a heater can raise to temperature every hour once the tank starts to drain. It is what lets a modest gas tank out-deliver a bigger electric one, and it is the second half of the first-hour-rating story.
Formula
Recovery is energy in, converted to heated gallons:
gas GPH = BTU/hr × efficiency ÷ (8.34 × ΔT)
electric GPH = kW × 3412 × efficiency ÷ (8.34 × ΔT)
The 8.34 is pounds per gallon of water; 3412 converts kW to BTU/hr. A bigger burner (more BTU/hr), higher efficiency, or a smaller temperature rise all raise the recovery rate.
Worked example
A 40,000 BTU/hr gas heater at 80% efficiency, raising water 70 °F:
GPH = 40,000 × 0.80 ÷ (8.34 × 70) = 32,000 ÷ 583.8 = 54.8 GPH
So it reheats about 54.8 gallons per hour. An electric element at 4.5 kW and the same rise recovers only about 26 GPH — which is why electric tanks need more stored volume for the same peak-hour delivery.
Background & practice
Recovery matters most for back-to-back demand: several showers in a row, or filling a large tub. A tank with a high recovery rate refills its hot supply quickly, so its first-hour rating exceeds its stored gallons. That is how a gas 40 can serve a household that an electric 50 would leave shivering.
Efficiency here is the fraction of fuel energy that reaches the water. Combustion gas tanks land around 0.75–0.82; heat-pump water heaters exceed 1.0 in effective terms because they move heat rather than make it — see the heat-pump savings tool. Enter your own coldest-season temperature rise for a realistic worst-case number.
Reference table
| Rise (ΔT) | Recovery at 40,000 BTU/hr, 80% eff |
|---|---|
| 60 °F | 63.9 GPH |
| 70 °F | 54.8 GPH |
| 80 °F | 48.0 GPH |
| 90 °F | 42.6 GPH |
A bigger burner, higher efficiency or a smaller temperature rise all raise recovery. Enter your own figures above.
Frequently asked questions
What is a good recovery rate for a water heater?
There is no single "good" number — it depends on your demand. A gas tank near 50–55 GPH at a 70 °F rise suits most families; electric tanks recover far slower (20–30 GPH), so they lean on stored volume instead.
Why is my gas heater faster than a bigger electric one?
Gas burners deliver far more BTU/hr than a standard electric element, so they reheat water much faster. That higher recovery rate lifts the first-hour rating above the tank’s stored gallons.
Does temperature rise change the recovery rate?
Yes — a larger rise means each gallon needs more energy, so gallons-per-hour drops. Winter groundwater is colder, so the same heater recovers fewer gallons per hour in January than in July.
How do I convert kW to BTU/hr?
Multiply kilowatts by 3412. A 4.5 kW element is about 15,350 BTU/hr of input, which is why electric tanks recover slowly compared with a 40,000 BTU/hr gas burner.