Water heater calculators

Questions people ask

What size water heater does a family of 4 need?

Size by the busiest hour, using the Department of Energy worksheet. Three showers at 10 gallons, a shave, four hand washes, a dishwasher load, food preparation and a load of laundry come to about 66 gallons. A 40-gallon gas tank with a 36,000 BTU burner has a first-hour rating of roughly 70 to 74 gallons and covers that. Electric tanks recover more slowly, so the same household usually needs a 65- to 80-gallon electric or heat pump unit; the first-hour rating on the EnergyGuide label decides between them.

How many GPM does a tankless water heater need for two showers?

Two showers at 2.0 GPM each need 4 GPM of hot water. With 50 °F groundwater and a 120 °F setpoint, the temperature rise is 70 °F and the heater must deliver 4 × 500 × 70 = 140,000 BTU per hour, which is a mid-size gas unit; an electric unit for the same job would draw about 41 kW, more than most homes can supply. In the southern United States, where groundwater is 70 °F, the same two showers need only 100,000 BTU per hour.

How long does a water heater take to heat up?

Recovery depends on the burner or element size. A 40-gallon gas tank with a 40,000 BTU input and 80% efficiency raises its water 70 °F in about 44 minutes; a 50-gallon electric tank with a 4,500-watt element takes about 1 hour 56 minutes for the same rise, because 4,500 W is only 15,350 BTU per hour. The recovery time calculator does the sum for any tank and input.

How much does a water heater cost to run?

For a household using 64 gallons of hot water a day heated through 65 °F, the energy is about 34,700 BTU a day. At 2025 US average prices that is roughly $24 a month for natural gas in a 0.65 UEF tank at $1.50 per therm, $57 a month for a standard electric tank at 17 cents per kWh, and $15 a month for a heat pump water heater with a UEF of 3.5. Hot water is typically the second largest energy use in a home after space conditioning.

How much does it cost to replace a water heater?

HomeAdvisor puts the average 2025 replacement at $1,333, with most homeowners paying $882 to $1,810. By type, a like-for-like gas tank costs about $900 to $1,800 installed, an electric tank $700 to $1,500, a gas tankless unit $2,500 to $4,500 (plus $1,000 to $2,500 when converting from a tank), and a heat pump water heater $1,500 to $4,000 before rebates. Changing fuel or moving the heater adds plumbing, electrical and venting work.