Water Heating Cost Calculator

How to use the water heating cost calculator

Enter how many gallons of hot water your household heats in a day, the temperature rise that water needs, the energy source, and your electricity rate, and you get back the monthly cost of heating water — plus the kilowatt-hours behind it each day and the annual figure. The defaults describe a fairly ordinary house: 80 gallons a day, a 70°F rise, electric-resistance heating, and a rate of $0.175/kWh, which lands at about $71.86 a month. Swap in your own numbers to see what your hot water actually costs.

Gallons per day is your whole household’s hot-water draw — showers, dishes, laundry, and the hand-washing in between. A long shower is roughly 15–20 gallons, a dishwasher cycle a few, a clothes washer on warm a good deal more, so 80 gallons a day is a reasonable two-to-three-person estimate. You are counting hot water that the heater had to warm, not total water, so cold-only taps and toilet flushes do not enter the figure. If in doubt, start with the default and adjust once you have watched a few bills.

Temperature rise is the gap between the water coming into your home and the temperature you store it at. Cold mains water arrives at roughly 50°F in much of the country, and a common tank setpoint is about 120°F, so the heater lifts each gallon about 70°F — that is the default. If your incoming water is colder in winter or your tank runs hotter, the rise grows and so does the cost; this single number, not the setpoint alone, is what drives the energy used, because heating to 120°F from 60°F is cheaper than heating to 120°F from 40°F.

Choose the energy source that matches your heater. “Electric resistance (COP 1)” is the plain electric element in a standard tank — every kilowatt-hour of electricity becomes one kilowatt-hour of heat. “Heat pump water heater (COP 3)” moves heat from the surrounding air instead of making it, so it delivers about three units of heat per unit of electricity and runs at roughly a third of the cost — about $23.95 a month on the same defaults. If you heat with gas, pick that option as a pointer: gas is sold per therm, not per kWh, so the natural-gas-cost-calculator and the gas-vs-electric-heating-calculator are the right tools for that math.

For the rate, use your own bill. The US residential average is about 17.5¢/kWh as of 2026, but it varies widely by state, so do not lean on the default if you have a real number — divide a full month’s total by the kilowatt-hours used, or add the supply and delivery line items together, and enter that. Read the monthly cost as the headline, the daily kWh when you want the energy free of any rate assumption, and the annual figure when you are sizing hot water against the rest of your power bill. Water heating is roughly 18% of a typical home’s energy use, so it is worth getting right.

The formula

The math is just physics turned into money. Water weighs a fixed amount per gallon and takes a fixed amount of heat to warm by a degree, so the gallons and the temperature rise give you the heat needed; converting that heat to electricity and multiplying by your rate gives you the cost:

BTU per day = gallons × 8.34 × temp rise
kWh per day = BTU ÷ 3412 ÷ COP
monthly cost = kWh per day × rate × 30

Worked example with the defaults — 80 gallons × 8.34 × 70°F = 46,704 BTU a day; ÷ 3412 = 13.69 kWh a day on electric resistance (COP 1); × $0.175 = $2.40 a day, or about $71.86 a month. With a heat-pump water heater (COP 3) the same hot water needs about a third of the electricity, landing near $23.95 a month.

The two constants are honest physics: a US gallon of water weighs 8.34 pounds, and it takes 1 BTU to raise 1 pound of water by 1°F, which is why BTU = gallons × 8.34 × °F rise. Then 3412 BTU is about 1 kWh, so dividing by 3412 turns the heat energy into the electricity that delivers it. A heat-pump water heater has a COP around 3 because it pumps existing heat rather than generating it, cutting the electricity to roughly a third.

Frequently asked questions