Water Heater Recovery Time Calculator
Water heater recovery rate and reheat time for electric, gas and heat-pump tanks: element kW or burner BTU/h, tank size, temperature rise or inlet and setpoint temperatures (°F or °C), part-tank reheats, estimated first-hour rating, energy and cost per reheat, and a recovery table by season.
How it works
Heat to water (BTU/h) = kW × 3,412 × efficiency (electric) = burner BTU/h × recovery efficiency (gas) = compressor kW × COP × 3,412 (heat pump) · Recovery (gal/h) = heat ÷ (8.34 × ΔT °F) — metric: L/h = kW × 860 ÷ ΔT °C · Reheat time = gallons × share used ÷ recovery · First-hour rating ≈ 0.7 × tank + 1 h of recovery · Energy per reheat = gallons × 8.34 × ΔT ÷ efficiency (or ÷ COP)
Heating water is pure energy accounting. It takes one BTU to raise one pound of water by 1°F, and a gallon weighs 8.34 pounds. In metric, 1 kWh heats 860 litres by 1 °C. An electric element turns almost all of its power into heat, at 3,412 BTU per hour per kilowatt. A gas burner loses some heat up the flue, so its input is multiplied by the recovery efficiency on the spec sheet. A heat pump moves heat from the room air and delivers its compressor power times the COP. The recovery rate is that heat divided by the energy each gallon needs, so it falls as the temperature rise grows. Cold winter inlet water is why a shower runs cold sooner in January than in July. That is why the table shows recovery from a summer to a cold-winter rise. Reheat time is the gallons you drew divided by the recovery rate. The first-hour rating adds the roughly 70% of the tank you can draw before it runs lukewarm to one hour of recovery. It is the figure to compare when sizing a heater for a busy morning.
Worked example
A 4.5 kW element at 98% efficiency delivers about 15,048 BTU/h. Heating water through a 70°F rise needs 8.34 × 70 = 584 BTU per gallon, so it reheats about 25.8 gallons per hour. A 50 gallon tank therefore recovers fully in about 1 hour 56 minutes, or 58 minutes if you drew only half of it. The estimated first-hour rating is 35 + 25.8 ≈ 61 gallons. A 40,000 BTU/h gas burner at 76% puts 30,400 BTU/h into the same tank. That is 52 gallons per hour, a full recovery in about 58 minutes. A heat pump alone (0.5 kW × COP 3) manages under 9 gallons per hour.
Sources: A. O. Smith — water heater sizing guide: recovery formulas and 70% usable draw · Engineering ToolBox — domestic hot water heater sizing and recovery · A. O. Smith (hotwater.com) — how long a water heater takes to reheat · U.S. Department of Energy — sizing a new water heater (first-hour rating)
Frequently asked questions
Does this work for gas water heaters?
Yes. Choose Gas and enter the burner input from the rating plate (for example 40,000 BTU/h) with the recovery efficiency, about 75–80% for a standard atmospheric tank. Gas tanks usually recover two to three times faster than electric ones of the same size.
Why does a bigger temperature rise slow recovery?
Recovery is inversely proportional to the temperature rise. Reheating through a 90°F rise instead of 70°F puts about 29% more energy into each gallon, so the same heater warms proportionally fewer gallons per hour. The table shows your heater from summer to cold-winter inlet temperatures.
Is first-hour rating the same as recovery?
No. The first-hour rating adds the hot water already stored, about 70% of the tank is usable, to one hour of recovery. This calculator estimates it, but the tested figure on the EnergyGuide label is the one to compare when shopping.
Why is my electric heater slower than two elements suggest?
Standard residential tanks wire the upper and lower elements so that only one runs at a time. A tank with two 4.5 kW elements still heats at 4.5 kW. Enter one element unless your heater is wired for simultaneous operation.
How slow is a heat pump water heater?
In heat-pump-only mode, a typical unit puts about 1.5 kW into the water, so it recovers roughly 8–10 gallons per hour at a 70°F rise. It uses about a third of the electricity of an element, which is why hybrid models switch on their element when you need hot water fast.
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This calculator is for educational purposes. Double-check important results before acting on them.