Electricity Cost Calculator

How to work out what an appliance costs to run

Your electricity bill charges for energy in kilowatt-hours (kWh). To price one appliance you need three things:

• Power in watts. Printed on the rating label or nameplate (often on the back or underneath), or in the manual. If only amps are given, watts ≈ volts × amps (120 V in the US, 230 V in most of Europe).
• How long it runs. Hours per day on the days it is used, and how many days a week.
• Your price per kWh. Divide the energy charges on your bill by the kWh used, so that delivery and other per-kWh charges are included. Rates differ a lot between states, countries, suppliers and tariffs, and some plans charge more at peak times, so use your own figure rather than the example.

The calculator shows the energy and cost per day, per average month and per year. Add appliances to see which ones dominate a bill; the table ranks each one’s share. For a car’s running cost instead, see the fuel cost calculator.

Electricity cost formula

watts

the power the appliance draws while running (1 kW = 1,000 W)

kWh

kilowatt-hour: 1,000 watts for one hour, the unit on your bill

days per year

days per week × 365 ÷ 7, so 7 days a week is 365 days

Dividing by 1,000 turns watt-hours into kilowatt-hours. A month here is an average month, one twelfth of a 365-day year (about 30.4 days), so twelve months add up to the yearly figure. Costs are rounded to the cent per appliance, and the totals are the sums of those rounded amounts.

Worked example

A 1,500-watt space heater runs 4 hours a day, every day, and electricity costs $0.15 per kWh.

1. Energy per day: 1,500 × 4 ÷ 1,000 = 6 kWh
2. Energy per year: 6 × 365 = 2,190 kWh (182.5 kWh a month)
3. Cost per year: 2,190 × $0.15 = $328.50
4. Cost per month: $328.50 ÷ 12 = $27.38, or $0.90 a day
5. Running it for one hour costs 1.5 kW × $0.15 = $0.225.

Add a 150 W refrigerator counted as running around the clock and the total becomes 3,504 kWh and $525.60 a year, with the heater making up 62.5% of it. In practice the refrigerator would cost less, because its compressor cycles on and off (see below).

Nameplate watts, actual draw and standby power

• The label shows the maximum. The wattage on a nameplate is what the appliance draws at full power. Many devices use less most of the time: a refrigerator or air conditioner cycles its compressor, a heater with a thermostat switches off when the room is warm, and a computer draws far less when idle. Using the nameplate figure for every hour overstates the cost.
• Measure when you can. A plug-in electricity usage monitor shows the real draw and the kWh used over a day or a week, which is more accurate than any estimate. Enter its average watts, or its kWh per day divided by 24 as the power with 24 hours of use.
• Standby power adds up. Televisions, game consoles, chargers and anything with a clock or remote control can draw power when “off”. Add a row with the standby watts and the hours spent on standby — for example 24 minus the hours in use — to include it. Unplugging or using a switched power strip removes it.
• Your rate may not be flat. Time-of-use tariffs, tiered prices and fixed monthly charges mean one average rate is an approximation. Fixed charges are not affected by how much you use.