Four people using 20 gallons of hot water a day each need 80 gallons a day and about 48 gallons in the busiest hour. A 4,500 W electric heater recovers 26 gallons an hour at a 70 °F rise, so a 40 gallon tank delivers about 54 gallons in the first hour — enough. It heats a full tank in 1.5 hours and uses 14.9 kWh a day at UEF 0.92. The calculator above sizes the tank for any household, electric or gas.
How the calculator works
Enter the people in the house, the hot water per person, the incoming water temperature and the thermostat setting, then pick electric or gas and its power. The result is the daily and peak-hour demand, the recovery, the tank and its first-hour rating, the time to heat it and the energy a day.
Tank by household, 20 gallons a person
| People | Peak hour | Electric, 4,500 W | Gas, 40,000 Btu/h |
|---|---|---|---|
| 1–3 | 12–36 gal | 30 gal | 30 gal |
| 4 | 48 gal | 40 gal | 30 gal |
| 5 | 60 gal | 50 gal | 30 gal |
| 6 | 72 gal | 75 gal | 30 gal |
Incoming water 50 °F, thermostat 120 °F.
Why gas tanks are smaller
A gas burner of 40,000 Btu/h at 80 % puts 32,000 Btu/h into the water — more than twice a 4,500 W element (15,400 Btu/h). It recovers about 55 gallons an hour, so even a 30 gallon tank covers the peak of a family of four. Electric tanks rely on stored water and are bought a size or two larger.
Energy a day
Heating a gallon by 1 °F takes 8.34 Btu. Energy a day = gallons × 8.34 × rise ÷ UEF, where the Uniform Energy Factor on the EnergyGuide label includes the standby losses: 0.92 for a standard electric tank, about 0.64 for gas, 3–4 for a heat pump water heater. Enter your price per kWh or therm to see the monthly cost.
A 4,500 W element on 240 V draws 18.75 A, a continuous load: see the Wire Size Calculator for the wire and breaker. Room heating: Heating BTU Calculator.