Appliances use different amounts of electricity. However, their power alone does not tell you what they cost to run. You also need to know how long you use them.
Power and Energy
First, it helps to know the difference between power and energy.
Power tells you how quickly an appliance can use electricity.
It is measured in:
- watts (W)
- kilowatts (kW)
Energy tells you how much electricity is actually used over time.
It is measured in:
- kilowatt-hours (kWh)
Your electricity bill charges you for the energy you use in kWh.
Watts and Kilowatts
Appliance power is often shown in watts.
There are 1,000 watts in one kilowatt.
So:
1,000 W = 1 kW
2,000 W = 2 kW
500 W = 0.5 kW
To change watts into kilowatts, divide by 1,000.
Work Out Energy Use
Once you know the power, you also need to know how long the appliance runs.
The basic calculation is:
Power in kW × time in hours = energy used in kWh
For example, a 2 kW heater running at full power for one hour would use:
2 kW × 1 hour = 2 kWh
If it ran at full power for two hours:
2 kW × 2 hours = 4 kWh
So, both power and time matter.
Work Out the Cost
Next, multiply the energy used by your electricity unit rate.
The calculation is:
Energy used in kWh × unit rate = cost
Suppose your electricity unit rate is 25p per kWh.
A 2 kW heater running at full power for one hour would use 2 kWh.
So:
2 kWh × 25p = 50p
The estimated cost would be 50p.
The rate above is only an example. So, use the unit rate from your own tariff when checking your costs.
What About Minutes?
Many appliances run for less than an hour.
So, first change the minutes into part of an hour.
Use:
Minutes ÷ 60 = hours
For example:
30 minutes = 0.5 hours
15 minutes = 0.25 hours
6 minutes = 0.1 hours
Then, use the normal calculation.
Kettle Example
Suppose a kettle is rated at 3 kW and takes three minutes to boil.
First, change three minutes into hours:
3 ÷ 60 = 0.05 hours
Then, work out the energy used:
3 kW × 0.05 = 0.15 kWh
At an example unit rate of 25p per kWh:
0.15 × 25p = 3.75p
So, one three-minute boil would cost about 4p in this simple example.
Heater Example
Now consider a 2 kW electric heater.
If it ran at full power for three hours:
2 kW × 3 hours = 6 kWh
At 25p per kWh:
6 × 25p = £1.50
So, the estimated cost would be £1.50.
However, many heaters do not draw their full rated power all the time. For example, a thermostat may switch the heating element on and off.
Therefore, the real cost may be different.
High Power Does Not Mean High Cost
A high-power appliance is not always the most costly appliance to use.
For example, a kettle uses a lot of power. However, it normally runs for only a few minutes.
Meanwhile, a lower-power appliance may run for many hours.
So, always consider power and time together.
Find an Appliance’s Power
You can often find the power rating on:
- the appliance label
- the rating plate
- the instruction book
- the maker’s website
The figure may be shown in watts or kilowatts.
However, the stated power does not always tell you exactly how much electricity the appliance will use.
Some appliances switch on and off. Others change their power while running.
Therefore, calculated running costs should usually be treated as estimates.
Use the Calculator
You do not need to work everything out yourself.
Enter the appliance power, how long you use it and your electricity unit rate into our calculator.
It will then estimate the electricity used and the running cost.
Use the Appliance Cost Calculator →
What About Standing Charges?
You do not need to add the standing charge when working out the extra cost of running one appliance.
You normally pay the standing charge each day whether that appliance is on or off.
So, for an appliance running-cost estimate, use the electricity unit rate.
Learn more: Energy Charges Explained →
Key Points
First, find the appliance power in watts or kilowatts.
Then, work out how long you use it.
Next:
Power × time = energy use
Then:
Energy use × unit rate = cost
However, remember that many appliances do not run at full power all the time. So, the result may only be an estimate.
Most importantly, don’t judge an appliance by its wattage alone. How long you use it matters too.
Next: Appliance Cost Calculator →
