Estimate the electricity cost of an electric water heater per use, per day, per month, and per year — by simple time-in-use or physics-based heating.
📂 Energy & Home CalculatorsFor a quick estimate, use simple mode: enter your heater's power in watts and roughly how many minutes it runs per use. For a more precise, physics-based figure — useful when sizing a tankless or on-demand heater, or estimating a shower's real energy cost — switch to advanced mode and enter the water volume, starting and target temperatures, and your heater's efficiency. Either way, add your uses per day and price per kWh, then press Calculate.
Electric water heaters are one of the largest energy draws in many homes, so this tool offers two ways to estimate their running cost depending on what you know. Simple mode treats the heater like any other appliance: power multiplied by time in use. Advanced mode instead calculates the actual physical energy required to raise a given volume of water from a starting temperature to a target temperature, using the well-known relationship Q = m × c × ΔT (mass × specific heat capacity of water × temperature change), then accounts for the heater's real-world efficiency, since no heater converts 100% of its electrical input into usable heat.
The specific heat capacity of water — how much energy it takes to raise its temperature — is a fixed physical property: 4,186 joules to raise 1 kilogram of water by 1°C. Since 1 liter of water has a mass of almost exactly 1 kilogram, this calculator can convert a water volume and a temperature rise directly into an energy requirement in joules, then into kilowatt-hours (1 kWh = 3,600,000 joules) for a cost estimate that isn't just guesswork.
Heater efficiency matters because not every joule of electricity that enters a water heater ends up as heat in the water — some is lost to the surrounding air, the tank's own walls, or (for tankless units) standby losses. A well-insulated electric tank heater is often 90% efficient or higher, which is why 90% is used as this calculator's default, but older or poorly insulated units can run somewhat lower.
Simple mode is the more practical choice when you already know roughly how long your heater runs per use — for a shower, that's easy to time. Advanced mode is more useful when you want to understand the underlying energy cost of heating water itself, independent of any particular heater's habits, such as when comparing a shower's water-heating cost against other household energy uses, or estimating the energy needed for a specific volume like a bathtub fill.
Water's density is almost exactly 1 kilogram per liter at typical household temperatures, so the two are used interchangeably in this kind of calculation without meaningfully affecting accuracy.
90% is a reasonable default for a modern, well-insulated electric tank heater. Older units, or ones with poor insulation or standing pilot losses, may run somewhat lower — try 80% for a more conservative estimate.
Simple mode is usually easier if you can time how long the water runs. Advanced mode is more useful if you want to know the actual energy needed to heat that water regardless of how efficiently a particular heater delivers it.