The formula
How it works
See what an appliance costs to run, from its power rating, how long you use it, and your electricity price. The calculator converts watts into kilowatt-hours and shows the cost per day, month and year.
FAQ
Where do I find an appliance’s wattage?
Check the label on the device or its plug, or the manual. It is given in watts (W); if only amps and volts are listed, multiply them to get watts.
What is a kilowatt-hour?
A kilowatt-hour (kWh) is the energy of a 1,000-watt device running for one hour, and it is the unit your electricity bill charges by. A 100 W bulb on for 10 hours uses exactly one kWh.
Where do I find my price per kWh?
Check a recent electricity bill — it usually lists a rate per kWh or gives you enough detail (total cost divided by total kWh) to work it out. Rates vary by provider, region and time of day, so use your own bill for the most accurate figure.
Does standby power count towards the cost?
Yes — many devices draw a small amount of power even when switched off or idle, and that counts as hours of use too. If an appliance is always plugged in, use 24 hours to estimate its true yearly cost, including standby draw.
Why does my actual bill not match this estimate?
This calculator assumes constant power draw, but many appliances cycle on and off — like fridges and air conditioners — so their average power is lower than the label suggests. Bills also include fixed charges, taxes and tiered rates that this simple formula does not account for.
How can I reduce an appliance’s running cost?
Cut any of the three inputs: use a lower-power or more efficient model, run it for fewer hours, or shift use to a cheaper rate period if your plan has one. Even small reductions in daily hours add up significantly over a year.
Is amps × volts always the right way to get watts?
For simple resistive devices, yes, but for motors and electronics the actual power can be lower than volts times amps because of power factor. When available, use the wattage printed on the label rather than calculating it yourself.
About the electricity cost calculator
This calculator shows how much it costs to run an electrical appliance, using its power rating, how many hours a day it runs, and the price you pay per unit of electricity. Energy is billed in kilowatt-hours, but appliances are labelled in watts and used for varying amounts of time, so the true running cost is not obvious. Turning those figures into a daily, monthly and yearly cost makes it easy to see which devices are cheap and which are quietly expensive.
How to use it
Enter the appliance’s power in watts, the number of hours you use it each day, and your electricity price per kilowatt-hour from your bill. The calculator shows the energy used and the cost per day, month and year. For example, a 100-watt device used 5 hours a day at 15 cents per kWh costs about 8 cents a day, or roughly $27 a year. Try different hours or a more efficient appliance to see how the running cost changes.
The formula
The cost is the energy used times the price, , where is the power in watts, is the hours of use and is the price per kilowatt-hour. Dividing watt-hours by 1,000 converts them to kilowatt-hours, the unit on your bill. Multiply the daily cost by 30 for a month and by 365 for a year to see how small hourly amounts add up over time.
Where it is used
Households use it to find the appliances driving up their bills — often heaters, tumble dryers and old fridges — and to judge whether an efficient replacement is worth buying. It helps compare the running cost of electric versus gas, and to size and value solar panels or a battery. Businesses use the same maths to budget energy for equipment and lighting, and it is a core tool in any effort to cut energy use and carbon.