Water Heater Recovery Time Calculator

How it is calculated

The tool works in US units: tank volume in gallons, element power in kW, temp_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.

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.