Heating water takes a fixed amount of energy per gallon per degree — so the time to heat a tank is just that energy divided by the element’s power, and the cost is the energy times your rate. Both fall out of one calculation once you know the volume, the temperature rise and the element size.
How long it takes
Time = energy ÷ element power. Water takes about 8.34 BTU per gallon per °F (4.186 kJ per litre per °C), so a 40-gallon (150 L) tank warming from 70 °F to 140 °F (about a 40 °C rise) needs roughly 7 kWh — about 2 hours 20 minutes on a 3 kW element. Half-depleted, it recovers in about half that.
How much electricity
kWh = gallons × temperature rise (°F) × 8.34 ÷ 3,412, plus a little for losses while heating. That 40-gallon, 70 °F heat-up is about 7 kWh — at $0.17/kWh, around $1.20 per full cycle. Water heating is one of the largest energy uses in a typical home, which is why it is the first place to look for savings.
Water Heating Time & Cost Calculator
Enter tank volume, element kW and temperature rise for the heat-up time and cost per cycle.
What temperature to set
Store at 120–140 °F (55–60 °C): hot enough to suppress Legionella bacteria, not so hot that standing losses and scald risk climb. Every 10 °F (5 °C) higher adds noticeably to the bill. Fit a thermostatic mixing valve where code requires delivery below storage temperature.
Does a timer save money?
Yes, in most homes. The tank loses 1–2 kWh a day standing, and a timer stops the element replacing those losses all day when nobody needs hot water — heat once before the morning peak and once before the evening. Savings are biggest on well-used tanks and time-of-use rates; an insulating jacket compounds them on older tanks.
Why it takes longer than the math
Real heat-ups fight standing losses, incoming water colder than assumed (especially in winter), scale on the element, and a thermostat that cycles near the setpoint. The calculator’s efficiency factor covers typical losses, but an old, scaled element in winter can be noticeably slower — if heating time has crept up over the years, scale is the usual culprit.