How to use the water heater running cost calculator
- Enter the people in the home and the hot water each uses per day. The 15 gallons shown is an example.
- Choose Fahrenheit or Celsius, then enter the temperature of the water coming in and the heater setting.
- Choose electricity or natural gas and enter the efficiency from the heater label and the price from your bill.
- Read the energy used, the cost per day, month and year, and the cost of each gallon.
Formula
heat per day in BTU = gallons x 8.3454 pounds x temperature rise in F
electric energy in kWh = BTU / 3,412.142 / efficiency
gas energy in therms = BTU / 100,000 / efficiency
cost per day = energy x price
cost per year = cost per day x 365
One BTU raises a pound of water by one degree Fahrenheit. A month is a twelfth of 365 days.
A family of four on electricity
Four people using 15 gallons each heat 60 gallons a day by 65 F (55 to 120). That is 60 x 8.3454 x 65 = 32,547 BTU, or 9.538 kWh before efficiency. At 92% efficiency the heater uses 10.368 kWh a day, which at $0.16 costs $1.66 a day, $50.46 a month and $605.49 a year, about 2.76 cents a gallon.
A gas heater at 62% efficiency heating 60 gallons by 75 F (50 to 125) uses 0.606 therms a day. At $1.50 a therm that is $331.63 a year. The Celsius case 12.8 C to 48.9 C is a rise of 65 F.
What drives the cost
Three things set the cost: how much hot water you use, how much you heat it and the price of the fuel. Lowering the setting by 10 degrees reduces the temperature rise and the energy by the same share. Colder incoming water in winter raises the cost.
The price per unit is a number from your bill. The page does not assume any rate, so enter your own electric price per kWh or gas price per therm.
What the estimate leaves out
Heat lost from the tank while it sits, pipe runs and hot water you run while waiting for it to arrive are not counted unless your efficiency figure covers them. The efficiency on a label is measured under particular conditions, so real use can differ. Use the result as a clean baseline and expect a real bill to be higher.
Related energy tools
Size a heater with the Water Heater Size Calculator, add up the appliances in the Electricity Cost Calculator, and compare fuels in the Electric vs Gas Appliance Cost Calculator. The Water Bill Calculator covers the water itself.
Assumptions and limits
- Hot water use, temperatures and efficiency stay the same all year.
- Water weighs 8.3454 pounds a gallon and takes one BTU per pound per degree Fahrenheit.
- Prices are flat per unit, with no tiers or fixed charges.
- Standby losses, recirculation pumps and fuel taxes are not modeled.
Frequently asked questions
How much does it cost to heat water for a year?
Multiply the energy to heat your daily hot water by the fuel price and 365. A family using 60 gallons a day, heated 65 F by a 92% electric heater at $0.16 per kWh, pays about $605 a year.
How many BTU does it take to heat a gallon of water?
A gallon weighs 8.3454 pounds, and each pound takes one BTU per degree Fahrenheit. Heating a gallon by 65 F takes about 542 BTU.
Does a lower thermostat setting save money?
It reduces the temperature rise, so it reduces the energy in proportion. Check the safe and comfortable temperature for your home before you change it.
What efficiency should I enter?
Use the figure on the label of your heater. The 92% for electric and 62% for gas on the page are examples only.