Electricity Cost Calculator

Watts, kilowatt-hours and the bill

A watt is a rate of energy use. Run 1,000 watts for one hour and you have used one kilowatt-hour, which is the unit your utility bills in. So the whole calculation is watts, times hours, divided by 1,000, times your rate — everything else on this page is that same arithmetic arranged by day, month and year.

The rating plate is a maximum, not an average

Most appliances cycle. A refrigerator rated at 150 W does not draw 150 W continuously; its compressor runs perhaps a third of the time, so its real consumption is closer to a third of the nameplate figure. The same applies to anything with a thermostat — heaters, ovens, air conditioners, kettles. For those, either enter the effective running hours rather than the hours it is switched on, or use the annual kWh figure from the energy label if one is available.

Standby is small per device and large per house

Devices that are never truly off — set-top boxes, games consoles in rest mode, chargers, anything with a remote control — draw a few watts continuously. A single 5 W standby load costs little on its own, but a typical household has dozens, and unlike the appliance itself they draw around the clock. The standby line above prices that for one device across the hours it is not in use.

Your rate is probably not one number

The figure above defaults to the US residential average published by the Energy Information Administration, but a bill rarely reduces to a single rate. Many tariffs are tiered, charging more per kilowatt-hour above a monthly threshold, and time-of-use plans charge several times more during peak hours than overnight. There is also a fixed customer charge that does not change with consumption. To get your own effective rate, divide the total on your last bill by the kilowatt-hours it covered — that includes the fixed charge and the taxes, which is what you actually pay.

Where the money usually is

Heating, cooling and water heating dominate most bills, and anything that produces heat as its purpose — dryers, ovens, space heaters, kettles — draws far more than electronics. A 1,500 W space heater run for eight hours costs more per day than a laptop does in a month. Lighting has largely left the list since LEDs, which is worth remembering before spending an evening unplugging phone chargers: the arithmetic above will tell you which changes are worth making and which are a rounding error.

Frequently Asked Questions