Electricity Cost Calculator
Electricity cost is power multiplied by time, converted into the unit the bill actually uses, then priced. Almost every wrong answer comes from confusing the two quantities — watts and kilowatt-hours — before any arithmetic begins.
Cost per day
1.26
- Energy used per day
- 4.5
- Cost per hour of use
- 0.42
- Cost over a 30-day month
- 37.8
- Cost over a year
- 459.9
Values update as you type. This calculator covers the single scenario its formula assumes — see Common Mistakes for what it leaves out.
The formula this calculator uses
Cost = watts x hours / 1000 x rate per kWh
- W
- Power rating in watts — the rate energy is used, taken from the label
- h
- Hours of use — how long the appliance actually runs, not how long it is plugged in
- rate
- Price per kilowatt-hour — total charge divided by total kWh
- kWh
- Energy consumed — watts times hours divided by 1000, the quantity the bill charges for
How to check the result by hand
- 1
Find the power rating in watts
The label on the appliance states its power, usually in watts or kilowatts. Convert kilowatts to watts by multiplying by 1000, so a 1.5 kW heater becomes 1500 W. If the label gives only current and voltage, multiply them together, since power equals volts times amps.
- 2
Estimate the hours of actual use per day
This is how long the appliance runs, not how long it is plugged in. A fridge's compressor might run eight hours in twenty-four rather than the full day, and a heater runs only while the room is cold. Guessing high here inflates every figure that follows, so it is worth watching the appliance for a day.
- 3
Convert watts and hours into kilowatt-hours
Multiply the wattage by the hours and divide by 1000. For 1500 W over three hours that is 4.5 kWh. Omitting the division by 1000 is the single most common slip, and it inflates the result a thousandfold while still looking like an ordinary number.
- 4
Recover the price per kilowatt-hour from the bill
Divide the total charge by the total kWh for the same period. A bill of 84 for 300 kWh yields 0.28 per kWh, tax and standing charges included. Using the all-in effective rate keeps the estimate comparable with what you actually pay.
- 5
Multiply energy by rate, then scale to the period you want
4.5 kWh at 0.28 is 1.26 a day. Scale that daily figure to 37.8 for a thirty-day month and 459.9 for a year. Multiplying the daily cost is valid only while usage stays constant, which is a reasonable first approximation and a poor one across seasons.
For worked examples, common mistakes and the limits of this formula, read the full How To Calculate Electricity Cost page.