What does it cost to run an appliance? Watts, kWh and your bill

To find what an appliance costs to run, multiply its power in watts by the hours it runs, divide by 1,000 to get kilowatt-hours (kWh), then multiply by your price per kWh. A 2,000 W space heater running for 3 hours uses 2 × 3 = 6 kWh, and at 0.30 per kWh that costs 6 × 0.30 = 1.80. The same method works for any appliance, as long as you use the right wattage and the right hours.

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Watts measure power, kWh measure energy

A watt is a rate of use: how fast a device draws electricity while it is on. A kilowatt-hour is an amount of energy: 1,000 watts for one hour. Your bill charges you in kWh, not watts. A 100 W bulb left on for 10 hours uses 100 × 10 = 1,000 watt-hours, which is 1 kWh.

Energy also has other units. One kWh is 3.6 megajoules, and the unit converter handles that conversion if a product label uses joules.

The formula with examples

Use the same three steps every time:

1. Power in kW = watts ÷ 1,000.
2. Energy in kWh = power in kW × hours used.
3. Cost = kWh × your price per kWh.

Appliance	Power	Time used	Energy	Cost at 0.30 per kWh
Space heater	2,000 W	3 hours	6.0 kWh	1.80
Electric kettle	1,500 W	6 minutes (0.1 hours)	0.15 kWh	about 0.05
LED bulb	10 W	5 hours a day, all year	18.25 kWh a year	5.48 a year
Old incandescent bulb	60 W	5 hours a day, all year	109.5 kWh a year	32.85 a year

The kettle and bulbs are examples. Replace the wattage with the figure from your own appliance. Changing the bulb in the table from 60 W incandescent to 10 W LED saves about 27 a year at this price, if the two give similar light (check the brightness on the box).

Find the wattage on the appliance

The wattage is usually on a label or rating plate on the back or underneath the appliance, or in the manual. If only volts and amps are shown, multiply them: watts = volts × amps. Treat the figure as a maximum. A thermostat-controlled heater or a fridge compressor runs only part of the time, so the real energy use is lower than watts × hours.

Refrigerators: use the yearly kWh on the label

A fridge runs on and off all day, so multiplying its nameplate watts by 24 hours overstates the cost. The better figure is the estimated annual energy use, in kWh per year, on the EnergyGuide label that the FTC requires on most new appliances. The U.S. Department of Energy's example of an 18.1 cubic foot top-freezer model lists 362 to 404 kWh a year, depending on efficiency.

For a fridge that uses 400 kWh a year at 0.30 per kWh, the cost is 400 × 0.30 = 120 a year, or 10 a month.

Standby power

A device that is switched off but still plugged in can draw a small amount of power all the time. A device drawing 2 W on standby, all year, uses 2 × 24 × 365 = 17.52 kWh, which costs 5.26 a year at 0.30 per kWh. Each device is small, but a home with many of them can add up. Smart power strips and switching off at the wall are the simplest fix.

Where to find your price per kWh

Look at the energy section of your bill for a unit rate, often shown as a price per kWh, such as "energy charge" in the US or "unit rate" in the UK. Fixed charges, such as a UK standing charge or a US fixed or customer charge, are not per kWh, so leave them out of the calculation.

If you only have a total, subtract the fixed charges from the total electricity charges, then divide what is left by the kWh used. The result is your average price per kWh.

Time-of-use tariffs

Some tariffs charge different prices at different times of day, with a higher peak rate and a lower off-peak rate. The same appliance then costs different amounts depending on when it runs. A 2,000 W heater using 6 kWh costs 6 × 0.45 = 2.70 at a peak rate of 0.45 per kWh, and 6 × 0.15 = 0.90 at an off-peak rate of 0.15. Check your tariff's times and rates, then run the calculation for each rate.

Measuring instead of estimating

A plug-in energy monitor reads the kWh used by one appliance directly, and some smart meters show the same figures for the whole home. A measurement over a normal week beats any estimate, especially for appliances that cycle, such as fridges and heaters with thermostats.

Common mistakes

• Mixing up Wh and kWh. Watt-hours are 1,000 times smaller than kWh. Divide by 1,000.
• Forgetting the days. Multiply a daily cost by the number of days to get a monthly or yearly one.
• Using the nameplate for a fridge. Use the label's yearly kWh instead.
• Assuming a high wattage always costs more. Cost depends on hours as well. A 2,000 W heater for 10 minutes costs less than a 60 W lamp left on all night.

Quick answers

Is the wattage on the label the power the appliance uses? It is the maximum draw. Many appliances use less most of the time.

Does a bigger appliance always cost more to run? No. Hours of use often matter more than wattage. Compare the kWh figures, not the watts.

Can I check the whole calculation quickly? Yes. The electricity cost calculator uses the same method: enter each appliance's watts, hours and your price per kWh, and it gives the cost per day, month and year.