What a kilowatt-hour is
Your power company does not bill you for watts. It bills you for energy, measured in kilowatt-hours (kWh). One kWh is the energy a device rated at 1,000 watts uses in one hour. A 100 W TV needs 10 hours to use the same kWh, while a 3,000 W clothes dryer gets there in 20 minutes.
That is why both numbers matter: how much power an appliance draws, and how long it runs. A small device that runs all day can cost more than a big one you switch on for a few minutes.
The formula
Example: a 1,500 W heater running 8 hours a day uses 1,500 × 8 ÷ 1,000 = 12 kWh. At $0.17 per kWh that is $2.04 a day, $61.20 for a 30-day month and $734.40 for a year of such months. The year here is simply 12 months at the days per month you entered, so for a heater you only use in winter, count the months yourself.
Typical appliance costs
Rough figures for a 30-day month at $0.17 per kWh. Wattages are typical, not measured, and your own devices can be well above or below them.
| Appliance | Typical watts | Use per day | kWh / month | Per month |
|---|---|---|---|---|
| Space heater | 1,500 W | 8 h | 360 | $61.20 |
| Window AC | 1,000 W | 8 h | 240 | $40.80 |
| Fridge (average over the day) | 150 W | 24 h | 108 | $18.36 |
| TV | 100 W | 4 h | 12 | $2.04 |
| Gaming PC | 400 W | 4 h | 48 | $8.16 |
| Laptop | 50 W | 8 h | 12 | $2.04 |
| LED bulb | 10 W | 5 h | 1.5 | $0.26 |
| Microwave | 1,100 W | 15 min | 8.25 | $1.40 |
| Clothes dryer | 3,000 W | 1 h | 90 | $15.30 |
| Dishwasher | 1,800 W | 1 h | 54 | $9.18 |
Heating and cooling stand out because they draw a lot of power for many hours. A heating element in a dryer, oven or dishwasher also draws a lot, but usually for a shorter time, and it cycles on and off, so treat those rows as a rough upper estimate.
Standby power adds up
Many devices keep drawing a little power when they look switched off: TVs, game consoles, chargers, speakers, printers and anything with a clock or a remote. A single device in standby might use only a few watts, but it does so 24 hours a day. Enter the standby watts with 24 hours per day to see what it costs you over a year.
- Use a power strip with a switch for the TV corner or the desk, and turn the whole group off at night.
- Check the settings of consoles and smart TVs for an energy saving or eco mode that disables quick start.
- A plug-in energy meter shows the real draw, both while running and in standby, which is better than any typical figure.
How to use the electricity cost calculator
- 1Pick a typical appliance, or enter the watts from your device’s label. If the label shows amps and volts instead, switch to amps × volts.
- 2Enter how many hours a day it runs and on how many days a month, then the rate per kWh from your electricity bill.
- 3Read the cost per hour, day, month and year. Add the appliance to the list and repeat for other devices to see the total.
Frequently asked questions
How do I calculate the electricity cost of an appliance?
Multiply the power in watts by the hours it runs, divide by 1,000 to get kilowatt-hours (kWh), then multiply by your rate per kWh. A 1,500 W space heater running 8 hours uses 12 kWh; at $0.17 per kWh that is $2.04 a day, or $61.20 over 30 days.
Where do I find the wattage of a device?
Look at the label or plate on the back or bottom of the device, the power adapter, or the manual. If the label only shows amps and volts, switch the calculator to amps × volts and it works out the watts for you. A plug-in energy meter shows what the device really draws.
Why is my fridge cheaper than its wattage suggests?
A fridge compressor switches on and off all day, so the label wattage is not what it draws on average. That is why the preset uses a rough average of 150 W over 24 hours. Many fridges list their yearly use in kWh on an energy label, which is the better number to start from.
Which electricity rate should I use?
Use the total price per kWh from your bill, including delivery charges and taxes if they are billed per kWh. Divide the total of those charges by the kWh you used that month for a fair average. Fixed monthly fees do not change with use, so leave them out.
Does this work for solar or time-of-use plans?
Yes, as a rough estimate. Enter the rate you pay during the hours the appliance runs. If it runs across peak and off-peak hours, use an average weighted by how long it runs in each, or work out each part separately and add them up in the list.