What is a kilowatt-hour, and how do you work out what an appliance costs?

A kilowatt-hour (kWh) is a unit of energy: the amount used by something drawing 1,000 watts for one hour. It is the unit electricity is sold in, so your bill is essentially the number of kWh you used multiplied by the price per kWh, plus any fixed charges. A 100 W device running for 10 hours uses 1 kWh, and so does a 2,000 W heater running for 30 minutes. The electricity cost calculator turns watts, hours and your tariff into a daily, monthly and yearly cost, and this guide explains the unit behind it.

What a kilowatt-hour is

Energy is power multiplied by time. The watt is a unit of power, a rate of energy use, and one kilowatt is 1,000 watts. Run one kilowatt for one hour and you have used one kilowatt-hour. In scientific units that is 3.6 million joules (3.6 MJ), because a watt is one joule per second and an hour has 3,600 seconds. Joules are too small for household bills, so electricity companies use kWh, and meters count them.

Watts vs kilowatt-hours

Watts tell you how hard something is working at a moment; kilowatt-hours tell you how much energy it used over time. The distinction matters because a high-wattage appliance used briefly can use less energy than a low-wattage one left on all day. A 3,000 W kettle boiling for 15 minutes a day uses 0.75 kWh. A 10 W router running 24 hours a day uses 0.24 kWh a day, about 88 kWh a year. The kettle is still the bigger user here, but the gap is far smaller than the 300-fold difference in watts suggests. Which appliances actually use the most electricity ranks the usual suspects.

Working out running costs

The formula is: kWh = watts ÷ 1,000 × hours, and cost = kWh × price per kWh. At a tariff of 0.30 per kWh (use your own currency and rate):

• a 100 W device for 4 hours a day uses 0.4 kWh, costing 0.12 a day, about 3.65 a month and 43.83 a year;
• a 2,000 W space heater for 4 hours a day uses 8 kWh, costing 2.40 a day and about 73 a month;
• a 9 W LED bulb for 5 hours a day uses 0.045 kWh, under 5 a year.

The calculator uses an average month of 30.44 days and a year of 365.25 days, which is why a month is not exactly 30 times the daily figure. Its appliance presets load typical wattages and hours so you can see the pattern quickly: things that make heat (heaters, dryers, kettles, ovens) dominate, and electronics barely register. For switching bulbs, the LED savings calculator adds payback time, and the EV charging calculator applies the same kWh arithmetic to a car battery.

Reading your bill

An electricity bill usually has two parts. The unit rate is the price per kWh, multiplied by the kWh your meter recorded between two readings. The fixed charge, called a standing charge in the UK and often a service or customer charge in the US, is paid per day or per month regardless of use. Some tariffs have different unit rates at different times of day, which makes it worth running big loads off-peak. To check a bill, subtract the previous meter reading from the current one to get kWh used, multiply by the unit rate, then add the fixed charge for the number of days and any taxes. Use the unit rate, not the bill total divided by kWh, when estimating what one appliance costs.

Why label watts mislead

The power on an appliance label is usually a maximum. A washing machine pulls its full rating only while heating water; a fridge's compressor cycles on and off, so its real average draw is far below its label. That is why the calculator's fridge-freezer preset uses an averaged 40 W, about 350 kWh a year, rather than the compressor's peak. Many appliances also list annual kWh on an energy label, which is a better starting point than the wattage. For a definitive answer, a plug-in energy meter measures actual kWh over a week.

kWh and other energy units

One kWh is 3.6 MJ or about 3,412 British thermal units (BTU). Gas is sometimes billed in therms or cubic metres and sometimes converted to kWh on the bill, which makes it easy to compare with electricity per unit of energy. For physics-style conversions between joules, calories, BTU and kWh, the energy calculator has the full table. Solar panel output is also quoted in kWh, so the same numbers tell you how much of your use a solar array could cover.