Home energy · hot-water planning
Water-heater recovery, from first principles.
Estimate the time to raise a tank's water from its inlet temperature to a target temperature using the heater's input and efficiency.
Inputs
Recovery estimate
Formula and caveats
Heat required = gallons × 8.34 lb/gal × temperature rise in °F. The result is converted from Btu to kWh using 3,412 Btu/kWh, then divided by heater input and recovery efficiency to estimate time.
This treats the whole tank as a single batch. It excludes standby loss, mixing valves, draw patterns, recovery cut-in controls and flow restrictions. DOE water-heater tests normalize delivered energy around a temperature rise and report electrical energy using the 3,412 Btu/kWh conversion.
Source: U.S. DOE water-heater test procedure · EIA: 1 kWh = 3,412 Btu