Water Heater Electricity Cost Calculator

How it works

By definition one BTU raises one pound of water by 1 °F, and a gallon of water weighs about 8.33 lb. The heat a water heater must add each day is the gallons used times 8.33 times the rise from the cold-water inlet to the tank setting. Dividing by 3,412 BTU per kWh gives the electricity, before losses.

Losses enter in one of two ways. With element efficiency, the heat is divided by the element's efficiency (about 98 % for a resistance element) and a separate standby loss, the heat the tank leaks while it sits, is added per day. A UEF rating is the Department of Energy's measured whole-day efficiency, which already includes standby loss, so in that mode the heat is divided by the UEF and nothing is added. A heat-pump water heater has a UEF above 1 because it moves heat from the air.

The recovery rate is how many gallons an hour the element can heat through the same temperature rise: the element's heat output in BTU/h divided by the BTU each gallon needs.

Formula

rise (°F)        = set temperature − inlet temperature
BTU per day      = gallons × 8.33 lb/gal × rise
kWh per day      = BTU ÷ 3,412.14 ÷ element efficiency + standby kWh   (element basis)
kWh per day      = BTU ÷ 3,412.14 ÷ UEF                                 (UEF basis)
kWh per year     = kWh per day × 365;   cost = kWh × rate ÷ 100
recovery (gal/h) = element kW × 3,412.14 × efficiency ÷ (8.33 × rise)

Example

60 gallons a day heated from 55 °F to 120 °F (a 65 °F rise) needs 60 × 8.33 × 65 = 32,482 BTU, or 9.52 kWh of heat. At 98 % element efficiency plus 1 kWh of standby loss that is 10.71 kWh a day and 3,911 kWh a year.

At 18 ¢ per kWh it costs $1.93 a day, $58.66 a month and $703.90 a year. A 4.5 kW element at that rise recovers 27.8 gallons an hour. On the UEF basis with UEF 0.92 the same water takes 10.35 kWh a day, $679.82 a year.

Assumptions and limitations

• Water weighs 8.33 lb per gallon (998 kg/m³, water near 70 °F) and its specific heat is 1 BTU per lb per °F. The small change in density between cold and hot water is ignored.
• Hot water use, inlet temperature and set temperature are held constant all year. Inlet water is colder in winter than in summer, so real costs vary by season. The 15–20 gallons per person and the efficiency, UEF and standby defaults are typical or illustrative values; the standby loss of a particular tank depends on its size, insulation, location and pipework.
• The UEF basis follows the DOE rating, which is measured at a fixed test draw and temperature rise; at other draws and rises the real efficiency differs. A heat-pump water heater's UEF also changes with the air temperature around it.
• The rate is a single flat price per kWh; the 18 ¢ default is illustrative.
• This is an energy-cost estimate, not a sizing or installation calculation. Water heater installation and set temperature limits are governed by the manufacturer's instructions and local plumbing and electrical codes.

Frequently asked questions