Quick answer: Running cost = watts × hours ÷ 1000 × price per kWh. A 100 W appliance used 5 hours a day uses 0.5 kWh a day, or 15 kWh in 30 days, which costs $2.25 a month at $0.15 per kWh. A kilowatt-hour is the energy used by a 1,000 W appliance running for one hour.
Electricity bills show one number for the whole house, which makes it hard to see what each appliance costs. A heater left on or an old freezer can be much more expensive than you would guess. If you know the power of an appliance and how long it runs, you can work out its running cost.
Enter the power in watts, the hours of use each day and your electricity price per kilowatt-hour (kWh). It shows the cost per day, per month and per year, and how many kWh the appliance uses.
How the cost is worked out
Cost = kWh × price per kWh
A kilowatt-hour is the energy used by a 1000 W (1 kW) appliance running for one hour. A 100 W bulb running for 10 hours uses exactly 1 kWh.
Typical power ratings are: LED bulb 5 to 15 W, ceiling fan 50 to 80 W, laptop 30 to 60 W, TV 50 to 150 W, refrigerator about 100 to 200 W while running, microwave 800 to 1200 W, kettle 1500 to 2500 W, and air conditioner 1000 to 3500 W. Appliances with a thermostat, such as fridges, heaters and air conditioners, switch on and off, so use the average time they are actually running, not 24 hours. Your bill may also include fixed charges and taxes, which this does not.
A worked example
Example. A 100 W appliance used 5 hours a day for 30 days uses 100 × 5 ÷ 1000 = 0.5 kWh a day, which is 15 kWh a month. At $0.15 per kWh, it costs $0.075 a day, $2.25 a month and $27.00 a year.
Typical running costs
| Appliance | Power | Hours a day | Energy a month | Cost a month at $0.15 |
|---|---|---|---|---|
| LED bulb | 10 W | 5 h | 1.5 kWh | $0.22 |
| Ceiling fan | 75 W | 8 h | 18.0 kWh | $2.70 |
| Laptop | 50 W | 6 h | 9.0 kWh | $1.35 |
| Television | 100 W | 4 h | 12.0 kWh | $1.80 |
| Refrigerator (running time) | 150 W | 8 h | 36.0 kWh | $5.40 |
| Air conditioner | 1500 W | 6 h | 270.0 kWh | $40.50 |
| Electric kettle | 2000 W | 0.5 h | 30.0 kWh | $4.50 |
| Microwave | 1000 W | 0.25 h | 7.5 kWh | $1.12 |
Illustrative examples. Check the label on your own appliances and use your own price per kWh.
Common mistakes to avoid
- Assuming a fridge or air conditioner runs all day. They switch on and off, so use the hours they are actually running, not 24.
- Ignoring standby power. Devices that are always plugged in add up, and a plug-in energy meter shows the real figure.
- Forgetting fixed charges and taxes. The bill includes more than the energy used, so the real cost is a little higher.
Frequently asked questions
How do I calculate the cost of running an appliance?
Multiply its watts by the hours used, divide by 1000 to get kWh, and multiply by your price per kWh. A 100 W appliance for 5 hours is 0.5 kWh, which at $0.15 costs about 7.5 cents.
Where do I find my price per kWh?
On your electricity bill, listed as the unit rate or energy charge. Use the average if you have different day and night rates.
What if the appliance only shows amps and volts?
Multiply volts by amps to get watts. A 230 V appliance drawing 5 A uses about 1150 W.
Why is my actual bill higher?
Bills include standing charges, taxes, and appliances that run in standby. Devices that are always plugged in add up. A smart plug with an energy meter shows the real figure.
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Sources and further reading
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