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 per kWh, which lands at about $71.86 a month and $874.33 a year. Swap in your own numbers to see what your hot water actually costs. Water heating is worth this attention: the Energy Information Administration groups water heating with lighting and refrigeration at 25 percent of annual household energy use in its 2020 Residential Energy Consumption Survey.

Gallons per day is your whole household hot-water draw — showers, dishes, laundry, and the hand washing in between. A long shower is roughly 15 to 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 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. The relationship is strictly proportional: halve the gallons and you halve the cost, which is why a shorter shower shows up on the bill in a way that most conservation advice does not.

Temperature rise is the gap between the water coming into your home and the temperature you store it at, and it matters more than the setpoint alone. Cold mains water arrives at roughly 50°F in much of the country and a common tank setting is about 120°F, so the heater lifts each gallon about 70°F, which is the default here. If your incoming water is colder in winter or your tank runs hotter, the rise grows and the cost grows with it in direct proportion. Heating to 120°F from 60°F is genuinely cheaper than heating to 120°F from 40°F. The Department of Energy recommends 120°F as the setting for most households, which balances scald risk and bacterial growth against energy use, and estimates that dropping a tank from 140°F to 120°F cuts water heating energy by 6 to 10 percent.

Choose the energy source that matches your heater, because it changes the answer by a factor of three. Electric resistance at COP 1 is the plain element in a standard tank: every kilowatt-hour of electricity becomes one kilowatt-hour of heat, and nothing better is physically possible for a resistor. A heat pump water heater at COP 3 moves heat from the surrounding air instead of making it, so it delivers roughly three units of heat per unit of electricity and runs at about a third of the cost — near $23.95 a month on the same defaults. If you heat with gas, pick that option as a pointer only: gas is sold per therm rather than 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 Energy Information Administration reported a US residential average of 18.11 cents per kWh year to date for 2026.

The formula

The math is physics turned into money, and it rests on two constants that are close to being definitions. Water has a specific heat of 1 BTU per pound per degree Fahrenheit — that is essentially what a BTU means — and a US gallon of water weighs about 8.34 pounds at ordinary temperatures. Multiply gallons by 8.34 to get pounds, multiply pounds by the temperature rise to get BTU, and you have the heat the water demands, before any question of how you supply it.

Turning that heat into electricity takes one more division and one adjustment. Dividing BTU by 3,412 converts the heat into kilowatt-hours, since a kilowatt-hour is 3,412 BTU. Then dividing by the coefficient of performance accounts for the machine: a resistance element has a COP of 1 and can never do better, because it converts electricity to heat one-for-one, while a heat pump water heater at COP 3 moves about three units of heat for every unit of electricity it draws and so needs a third of the energy for the same result.

BTU per day = gallons × 8.34 lb per gallon × temperature rise in °F
kWh per day = BTU ÷ 3412 ÷ COP
daily cost = kWh per day × rate
monthly cost = daily cost × 30

Worked example with the defaults: 80 gallons × 8.34 lb × 70°F = 46,704 BTU a day. Divided by 3,412 that is 13.69 kWh a day on electric resistance, where COP is 1. At $0.175 per kWh that is $2.40 a day, about $71.86 a month, and $874.33 across a year. Switch to a heat pump water heater at COP 3 and the same 46,704 BTU needs only 4.56 kWh a day, costing about $0.80 a day and $23.95 a month — a saving of $47.91 every month for hot water that is indistinguishable at the tap.

Because both constants are fixed, every result scales cleanly and you can reason about changes without re-running anything. Halve the gallons and you halve the cost; halve the temperature rise and you halve it again. That is why the Department of Energy recommendation to set a tank at 120°F rather than 140°F is worth acting on: it removes 20 degrees from the rise, and the Department estimates the change cuts water heating energy by 6 to 10 percent, with a tank left at 140°F wasting an estimated $36 to $61 a year in standby heat loss alone.

Two things the calculation deliberately leaves out. It does not model standing losses — the heat a full tank sheds into the room hour after hour whether or not anyone draws water — so a real bill sits above this figure, and the gap is widest on an old, poorly insulated tank in an unheated basement. It also does not price gas, because gas is sold per therm rather than per kWh; the gas option in the selector is a pointer to the natural-gas-cost-calculator and the gas-vs-electric-heating-calculator rather than a calculation. Treat the output as the energy cost of the water you actually use, not as a bill prediction.

Frequently asked questions