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What a kilowatt-hour is, and how to work out what an appliance costs to run
Electricity bills are written in kilowatt-hours, appliances are labelled in watts, and most people never quite join the two up. Once you do, working out what a heater, a PC or a fridge costs to run is a two-line sum.
Watts and kilowatt-hours
A watt is a rate: how fast something uses energy right now. A 2000 W heater uses energy twice as fast as a 1000 W one.
A kilowatt-hour is an amount: the energy used by something drawing 1000 W for one hour. It is what the meter counts and what the retailer charges for.
The link between them is time:
kWh = watts × hours ÷ 1000
A 2000 W heater running for three hours uses 2000 × 3 ÷ 1000 = 6 kWh. A 10 W LED bulb would need to run for 100 hours to use one kWh.
The kilowatt-hour is also just a unit of energy, like the joule, but a much larger one. One kWh is 3.6 million joules, which is why energy bills do not use joules. The joules to kWh converter handles the conversion if you meet energy figures in joules or megajoules, as gas bills sometimes do.
From kWh to cost
Multiply the energy by your rate:
cost = kWh × price per kWh
Your rate is on your bill, usually in cents or pence per kWh. Prices vary a lot between countries, retailers, plans and times of day, so always use the number from your own bill. As an example only, at 30 cents per kWh:
| Appliance | Power | Hours/day | kWh/day | Cost/day |
|---|---|---|---|---|
| LED bulb | 10 W | 5 | 0.05 | 1.5c |
| Laptop | 60 W | 8 | 0.48 | 14.4c |
| Gaming PC under load | 450 W | 3 | 1.35 | 40.5c |
| Fan heater | 2000 W | 4 | 8.0 | $2.40 |
| Clothes dryer | 2400 W | 1 | 2.4 | 72c |
Heating and anything with a heating element dominate. A heater running all evening can cost more than every light, screen and charger in the house put together.
The label is a maximum
The wattage on an appliance's label is usually its maximum draw. Many appliances spend most of their time well below it:
- A fridge's compressor cycles on and off, so its daily use is far below its rated power × 24. The energy label's annual kWh figure is a better guide.
- A PC idling on the desktop draws a fraction of what it does in a game.
- A heater with a thermostat cuts out once the room is warm.
- A TV draws less on a dark scene than a bright one.
For a real figure, a plug-in power meter between the socket and the appliance measures actual use over a day or a week. It is the most reliable way to find out what something costs.
Standby adds up
Devices on standby draw small amounts all the time. A few watts each is nothing on its own, but a household can easily have a dozen devices doing it. Ten devices at 3 W each is 30 W around the clock:
30 W × 24 h × 365 ÷ 1000 = 263 kWh a year
That is worth knowing before you go round switching things off at the wall, and also worth keeping in proportion: one long hot shower or an evening of heating uses far more.
The supply charge
Most bills have two parts. The usage charge is the kWh × rate part above. The daily supply charge is a fixed amount for being connected, charged whether you use anything or not.
It does not change what an individual appliance costs to run, but it matters when comparing plans. A plan with a cheaper rate and a higher supply charge can cost more for a small household that uses little electricity. Work out a year at your actual usage on each plan rather than comparing rates alone.
Time-of-use plans add a further twist, with different rates at peak, shoulder and off-peak times. On those, running the dishwasher or charging a car overnight can cost noticeably less than the same job at dinner time.
Solar changes the sums
If you have solar panels, electricity you generate and use yourself is not bought from the grid, so its cost is the cost of the panels spread over their life. Exports earn a feed-in tariff, usually much lower than the price you pay for imports. That gap is why running appliances in the middle of the day matters so much for solar households: each kWh used directly saves the full import price instead of earning the smaller feed-in rate. The solar payback calculator works out how long a system takes to pay for itself from that split.
Work it out
The electricity cost calculator takes kWh used and your tariff, includes the daily supply charge, and shows cost per day, month and year, along with what the same usage would cost across a range of tariffs. For the rest of the household running costs, from water to phone plans, see the bills section.