How to Calculate Electricity Cost from Watts and kWh

The power company charges by the kilowatt-hour (kWh): 1,000 watts used for one hour. To find what an appliance costs to run, find how many kilowatt-hours it uses and multiply by the price.

For learning and checking coursework only. Electrical work can kill. Follow the electrical code where you live and have a licensed electrician do or inspect real wiring.

Try it with your own problem ↓

The steps

Find the power in watts (it is on the label) and change it to kilowatts: watts ÷ 1,000.
Energy = kilowatts × hours of use. That is kilowatt-hours.
Cost = kWh × price per kWh. Change cents to dollars first (12¢ = $0.12).
For a month, multiply the daily cost by 30; for a year, by 365.
Round the money to the cent at the end.

A worked example

A 1,500 W space heater runs 6 hours a day, and electricity costs $0.16 per kWh. What does it cost a day, a month and a year?

Kilowatts: 1,500 ÷ 1,000 = 1.5 kW.
Energy a day: 1.5 kW × 6 h = 9 kWh.
Cost a day: 9 × $0.16 = $1.44.
A month (30 days): $1.44 × 30 = $43.20. A year: $1.44 × 365 = $525.60.

About $1.44 a day, $43.20 a month and $525.60 a year.

Why it works

Power is a rate, like miles per hour, and energy is the total, like miles driven, so the rate times the time gives the total. Dividing by 1,000 puts the energy in kilowatt-hours, the unit the power company prices, so the price can be multiplied straight in.

Try it with your own problem

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Work it out: a 1,500 W heater, 6 hours a day, at $0.16 per kWhOpens The Circuit Board with this example

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Common mistakes

Forgetting to divide by 1,000. If you get 1,500 × 6 × 0.16 = $1,440 a day, the watts were never changed to kilowatts.
Using cents as dollars. 16 cents is $0.16, not $16.
Mixing up kW and kWh. kW is how fast energy is used; kWh is how much. A 1.5 kW heater running 6 hours uses 9 kWh.
Expecting the bill to match exactly. Bills add fixed charges and taxes, and a thermostat switches a heater on and off.

Questions people ask

What does a 60 W bulb cost 5 hours a day at 12 cents per kWh?

0.06 kW × 5 h = 0.3 kWh a day, and 0.3 × $0.12 = $0.036: about 4 cents a day, or $1.08 a month.

How many kWh does a 150 W fridge use in a day?

0.15 kW × 24 h = 3.6 kWh at most. A fridge’s motor runs only part of the time, so the real number is lower.

What if the label shows amps, not watts?

Watts = volts × amps. A 10 A appliance on 120 V uses 1,200 W.

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