How the Electricity Calculator Works
An electricity bill is built from one number: the kilowatt-hours (kWh) that you use. Every device on your bill uses a part of it. The Electricity Cost Calculator shows how many kilowatt-hours a device uses and what they cost, so you can find what is expensive to run.
What is a kilowatt-hour?
A watt is a measure of power, which is how fast a device uses energy. A kilowatt (kW) is 1,000 watts. A kilowatt-hour is the energy that a 1,000-watt device uses in one hour. It is the unit that your electricity company sells.
The formulas
energy per day (kWh) = power (watts) x hours per day / 1000 cost per day = energy per day x price of one kWh cost per year = cost per day x 365 cost per month = cost per year / 12
If you use the device only on some days, the calculator multiplies by the days per week divided by 7. If you have more than one device, it multiplies by the number of devices.
Worked example: a 100 W device, 5 hours a day, at $0.15 per kWh
| Step | Calculation | Result |
|---|---|---|
| Energy per day | 100 x 5 / 1000 | 0.5 kWh |
| Cost per day | 0.5 x 0.15 | $0.075 |
| Cost per year | 0.075 x 365 | $27.38 |
| Cost per month | 27.375 / 12 | $2.28 |
| Energy per year | 0.5 x 365 | 182.5 kWh |
The month has 365 / 12 = 30.4 days on average. This is why the calculator divides the year by 12, and it does not use a month of exactly 30 days, which would give a monthly cost that does not agree with the yearly one.
Chart: the effect of the hours of use
The same 100 W device at $0.15 per kWh. Each block is about $0.27 a month. The cost grows in a straight line with the hours.
1 h ██ $0.46 3 h █████ $1.37 5 h ████████ $2.28 8 h █████████████ $3.65 12 h ████████████████████ $5.48 24 h ████████████████████████████████████████ $10.95
Chart: the effect of the price of electricity
The same device for 5 hours a day. Each block is about $0.15 a month. Prices are very different from place to place, so use the one on your bill.
$0.10 ██████████ $1.52 $0.15 ███████████████ $2.28 $0.20 ████████████████████ $3.04 $0.30 ██████████████████████████████ $4.56 $0.40 ████████████████████████████████████████ $6.08
Examples of devices
These are example values at $0.15 per kWh. The real power of your device is on its label or in its manual.
| Device | Power | Use per day | kWh per day | Cost per day | Cost per month |
|---|---|---|---|---|---|
| LED bulb | 10 W | 5 h | 0.05 kWh | $0.01 | $0.23 |
| Laptop | 60 W | 8 h | 0.48 kWh | $0.07 | $2.19 |
| TV | 120 W | 5 h | 0.60 kWh | $0.09 | $2.74 |
| Window AC | 1000 W | 8 h | 8.00 kWh | $1.20 | $36.50 |
| Space heater | 1500 W | 4 h | 6.00 kWh | $0.90 | $27.37 |
Each block of the chart is about $0.9 a month. Green is under $2, amber is $2 to $15, and red is over $15.
LED bulb █ $0.23 (10 W, 5 h) Laptop ██ $2.19 (60 W, 8 h) TV ███ $2.74 (120 W, 5 h) Window AC ████████████████████████████████████████ $36.50 (1000 W, 8 h) Space heater ██████████████████████████████ $27.37 (1500 W, 4 h)
Devices that make heat (a heater, an oven, a kettle, a hair dryer) use a lot of power, so they cost the most even if you use them for a short time.
How to find the power of a device
- Read the label. The power in watts is often printed on the back or on the plug. If it shows amps and volts, multiply them to get watts.
- Use a plug-in power meter. It measures the real use, and it is the best way for a device that changes its power, such as a fridge or a computer.
- Remember the label is often the maximum. A fridge, an air conditioner or a heater with a thermostat turns on and off, so it uses less than its label times the hours in the day. Enter the hours that it really runs.
Ways to lower the cost
- Turn off what you do not use, and unplug devices that use power on standby.
- Choose efficient devices. LED bulbs use only a small part of the power of the old bulbs for the same light.
- Use heating and cooling with care. A few degrees of change in the thermostat can save a lot.
- Check your tariff. Some companies charge less at night or at other times of the day.
What this calculator does not include
- Fixed charges, taxes and fees on your bill, or a price that changes with the amount you use or the time of day.
- The changes in power when a device starts up, or when it cycles on and off.
- Solar panels, batteries and the price of a device.
This tool gives an estimate for general information. For your real cost, check your bill.