Electricity Cost Calculator

How to use the electricity cost calculator

Enter the power of the device in watts, how many hours a day it runs, over how many days, and your electricity rate, and the tool returns what it costs to run over that whole window, per day, and per 30-day month. The arithmetic is trivial and the accuracy lives entirely in the inputs, which is why three of the four fields deserve a moment of thought rather than a guess.

The rate is the input that varies most: the US residential average was 18.44 cents per kilowatt-hour in May 2026 according to the Energy Information Administration Electric Power Monthly released 23 July 2026, but the same table put Idaho at 12.35 cents and Hawaii at 52.00 cents in that single month. A national average is a starting point, never an answer.

Be equally honest about hours and days, because runtime is the lever that decides most of the answer. A space heater described as on all evening runs five or six hours, not twenty-four. A desktop left awake overnight runs sixteen. A refrigerator sits in the kitchen around the clock but its compressor works only a fraction of that time, which is exactly why the nameplate overstates it.

Set the days to match the window you are pricing, so 30 for a month and 365 for a year, and set the hours to the time the device is drawing power rather than the time it is plugged in. Standby draw is the exception that rewards a 24-hour entry: a device idling at 3 W all year uses 26.3 kWh, which is small alone and considerable across a house.

Should you enter nameplate watts or the real draw?

Watts can be the nameplate figure or the real draw, and the two are rarely the same number. The label on the back of a device states the maximum it is permitted to pull, and plenty of equipment never approaches that in normal use.

Anything thermostat-controlled is worse: a refrigerator, a window air conditioner, a freezer, or an electric water heater cycles its compressor or element on and off, so the average draw across an hour sits far below the plate. If your label lists only volts and amps, watts is volts times amps, so a 12 A appliance on a 120 V circuit is about 1,440 W.

For a figure you can defend, a plug-in energy monitor at the outlet reads the actual draw and, over a day, the actual kilowatt-hours.

The formula, worked line by line

Cost is energy times your rate, and energy is power times time. Watts are divided by 1,000 to become kilowatts so the units line up with the kilowatt-hours your utility meters and bills. A kilowatt sustained for one hour is one kilowatt-hour, and that single sentence is the whole of the physics on this page.

Everything difficult about this calculation sits upstream of the multiplication. Power has to be the real draw rather than the nameplate ceiling, hours have to be genuine runtime rather than time plugged in, and the rate has to include every per-kilowatt-hour charge on the bill rather than the supply component alone. Get those three right and the arithmetic below cannot be wrong.

kWh = (watts / 1000) x hours per day x days
cost = kWh x rate
daily cost = (watts / 1000) x hours per day x rate
volts x amps = watts   (when the label lists current, not power)

The lever is watts multiplied by hours, and the two are interchangeable in the result. A 1,000 W microwave used six minutes a day is 3 kWh a month and about 53 cents. A 50 W device left on permanently is 1.2 kWh a day, 36 kWh a month and $6.30 at the same rate, which is twelve times as much for a twentieth of the power.

This is why hunting for the biggest number on a nameplate is the wrong instinct: the bill is decided by the product, and always-on beats occasionally-enormous more often than people expect.

Questions people ask