How to use it
- Type one appliance per line, with its name, its power in watts and its hours a day.
- Type the Price per kWh from your bill and the Days in the month.
- Read Cost per month, then open Each appliance to see which appliances use the most.
How the energy and cost are worked out
An appliance uses its power multiplied by the hours it runs. Divide by 1 000 to get kilowatt-hours (kWh). A kilowatt-hour is what a 1 kW appliance uses in one hour.
The monthly energy is the daily energy times the days in the month, the yearly energy is the daily energy times 365, and each cost is the energy times the price per kWh on your bill.
kWh per day = watts × hours a day ÷ 1 000
kWh per month = kWh per day × days
kWh per year = kWh per day × 365
cost = kWh × price per kWhStandby power
A device on standby uses a small power all the time. The arithmetic is simple: 1 W on all day is 1 × 24 ÷ 1 000 = 0.024 kWh a day, which is 8.76 kWh a year. Enter it as a line with 1 watt and 24 hours to see its cost.
Read the standby power from the device's data sheet, or measure it with a plug-in meter; this page does not give standby figures for devices, because they differ by model.
What this calculator does not include
- The tariff structure: fixed charges, tiered rates, taxes and the levies that a bill adds to the energy price. Use the total of the bill divided by the kWh for the true average price.
- Heating and cooling that runs by thermostat, whose power changes over the day; for those, use the average power you measure.
Worked example: five appliances at 3.50 per kWh for 30 days
The refrigerator uses 120 × 24 ÷ 1 000 = 2.88 kWh a day, which is 86.4 kWh a month and 302.40 at 3.50 per kWh. The five appliances together use 4.22 kWh a day.
For the month that is 126.6 kWh and 443.10. For the year it is 1,540.3 kWh and 5,391.05.
| kWh a day | kWh a month | Cost a month | |
|---|---|---|---|
| Refrigerator | 2.88 | 86.4 | 302.40 |
| All five | 4.22 | 126.6 | 443.10 |
Frequently asked questions
How do I work out the cost of one appliance?
Multiply its watts by its hours a day, divide by 1 000 to get kWh, then multiply by the price per kWh. A 100 W television for 4 hours a day uses 0.4 kWh, which is 12 kWh a month.
Where do I find the price per kWh?
On your electricity bill, as the unit price per kWh for the energy you use. Some bills show several prices, for example for different levels of use, so choose the one that matches your usage.
What is a kilowatt-hour?
The energy of a 1 kW appliance running for one hour. A 1 000 W heater running for an hour uses 1 kWh; a 60 W light bulb running for 10 hours uses 0.6 kWh.
Is the power on the label the power it uses?
It is usually the maximum. Many appliances use less most of the time, so the real energy is often lower than the label suggests. A plug-in energy meter shows the real figure.
Does the tool store what I enter?
No. The calculation runs in your browser and nothing you type is sent to us or stored.
Sources
Figures and text last checked on Tuesday, November 10, 2026.