How do I convert watts to kilowatt-hours?

Multiply the power in watts by the number of hours it runs, then divide by 1000. For example, a 1000 W heater running for 1 hour uses (1000 × 1) ÷ 1000 = 1 kWh.

What is the difference between watts and kilowatt-hours?

Watts measure power — the rate at which a device draws electricity right now. Kilowatt-hours measure energy — the total amount used over time. A 2000 W kettle has high power but, run for only 3 minutes, uses very little energy.

How do I work out the cost from kWh?

Multiply the kWh result by your electricity unit price. If a fridge uses 1.2 kWh in a day and your rate is 30p per kWh, that is 1.2 × 0.30 = 36p per day.

How many kWh does a 1500 W heater use in 3 hours?

(1500 × 3) ÷ 1000 = 4.5 kWh. At a 30p unit rate that costs about 1.35 in your local currency for the three hours.

Should I use the rated wattage or actual draw?

Many appliances draw less than their nameplate rating most of the time, or cycle on and off (like fridges and air conditioners). For a realistic estimate use the average running wattage; for a worst case use the rated wattage.

Is this calculator accurate?

Yes. It uses the exact relation kWh = (watts × hours) ÷ 1000. The only approximation is your own input — real appliances vary their draw, so use the average or rated wattage for an estimate.

Is my data sent anywhere?

No. The calculator is privacy-first — every calculation happens locally in your browser and nothing you type is transmitted or stored.

Watts to Kilowatt-Hours (kWh) Calculator

Watts to Kilowatt-Hours Calculator

This tool converts an appliance’s power in watts and its running time in hours into the energy it consumes in kilowatt-hours (kWh) — the unit you are billed for. It is useful for estimating what a heater, kettle, computer, or EV charger costs to run before you ever see the bill.

How it works

Energy equals power multiplied by time. Because watts are a thousandth of a kilowatt, the formula divides by 1000 to land in kilowatt-hours:

kWh = (watts × hours) ÷ 1000

So a 1000 W appliance running for one hour uses exactly 1 kWh. To turn that into money, multiply the kWh figure by your electricity unit price.

Power	Run time	Energy
100 W	1 h	0.1 kWh
500 W	1 h	0.5 kWh
1000 W	1 h	1.0 kWh
2000 W	1 h	2.0 kWh

Example

A 1500 W electric heater running for 4 hours uses (1500 × 4) ÷ 1000 = 6 kWh. At a unit price of 30p per kWh, that is 6 × 0.30 = 1.80 for the session.

Enter the wattage and hours — the result updates in real time, and the reference table shows common wattages at your chosen run time. All calculations run entirely in your browser with no data sent to any server.