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Water Heater Recovery Time Calculator

Enter tank size, element power and the temperature rise to see how long until you have hot water again.

How it is calculated

The tool works in US units: tank volume in gallons, element power in kW, Temperature rise in °F and efficiency in percent. Water is taken as 8.34 lb per gallon with a specific heat of 1 BTU per lb·°F, and 1 kWh as 3,412.142 BTU.

kWh = volume × 8.34 × rise ÷ 3412.142
hours = kWh ÷ (power × efficiency ÷ 100)

The minutes figure is hours × 60, and the recovery rate shown is volume ÷ hours. Volume, power, rise and efficiency must all be above zero, efficiency cannot exceed 100, element power above 100 kW gives no result (a sign that watts were typed into the kW field), and a blank efficiency field gives no result because there is no default. Standing heat loss while heating is not modelled.

Worked example

A 50-gallon tank with a 4.5 kW element must be heated by 70 °F (say from 50 °F to 120 °F) at 90 % efficiency. Energy: 50 × 8.34 × 70 = 29,190 BTU, which is 8.55 kWh. Usable power: 4.5 × 0.90 = 4.05 kW. Time: 8.55 ÷ 4.05 = 2.11 hours, about 127 minutes. That equals a recovery rate of 23.7 gal/h.

Try this example →

Common mistakes

  • Entering the temperature rise in °C. The field expects °F, so a 40 °C rise (72 °F) typed as 40 gives a recovery time that is far too short.
  • Typing the target water temperature (120) instead of the rise (target minus incoming water temperature).
  • Entering the volume in litres. The tool expects US gallons, so a 200-litre tank is about 53 gallons.
  • Using the wattage in watts (4500) instead of kilowatts (4.5). Any element power above 100 kW is rejected, so 4500 gives no result; but a slip that stays under 100, such as 45 for 4.5, would look ten times faster.
  • Forgetting that a two-element tank usually heats with one element at a time. Enter the power of the element that is actually on.

About this calculator

Recovery time is how long a water heater needs to reheat its full tank after being drawn down, and it depends on the tank size, the heating element's wattage and the temperature rise required: the energy needed equals the water volume times its specific heat times the temperature rise, and dividing that by the element's power output (after accounting for typical heating efficiency) gives the time in hours. A higher-wattage element or a smaller required temperature rise both shorten recovery time.

Plumbers and homeowners use this calculation when sizing a replacement heating element or comparing water heater models, since a household that regularly runs out of hot water mid-shower usually has a recovery time problem rather than a tank-size problem — a bigger tank does not help if the element cannot reheat it fast enough between uses.

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