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.
Common appliance wattages to try
Appliance wattages vary by model, age, and efficiency rating, but typical ranges give a useful starting point:
| Appliance | Typical wattage |
|---|---|
| LED bulb | 6–10 W |
| Laptop | 30–65 W |
| Desktop PC (gaming) | 150–400 W |
| Refrigerator | 100–200 W |
| Washing machine | 500–2,000 W |
| Tumble dryer | 2,000–5,000 W |
| Electric kettle | 1,500–3,000 W |
| Electric shower | 7,000–10,500 W |
| EV home charger (Level 2) | 3,700–22,000 W |
The wattage is almost always printed on a label on the appliance itself, or listed in the manual and product specification. Use the nameplate wattage for a worst-case estimate; some appliances draw less in normal operation.
Why watts and kWh are different units
A common source of confusion is treating watts and kilowatt-hours as interchangeable. They measure different things:
- Watts measure power — the rate at which electricity is being drawn right now. A kettle drawing 2,000 W is consuming electricity at a rate of 2 kW at that instant.
- Kilowatt-hours measure energy — the total electricity used over a period of time. The same kettle, run for 3 minutes (0.05 hours), uses 2 × 0.05 = 0.1 kWh — a tenth of a unit.
Your electricity bill is in kWh (energy), not watts (power). That is why running time matters: a high-wattage appliance used briefly can cost less than a low-wattage device left on continuously.
Converting for different time periods
The formula uses hours, but it is straightforward to adjust:
- Minutes: divide minutes by 60 first to get hours. For example, 15 minutes = 0.25 hours.
- Days: multiply hours per day by the number of days.
- Months: multiply daily kWh by 30 for an approximate monthly figure.
A 100 W device left on all day (24 hours) uses (100 × 24) ÷ 1000 = 2.4 kWh per day — over a month that is roughly 72 kWh, worth paying attention to at higher electricity rates.
Enter the wattage and hours — the result updates in real time. All calculations run entirely in your browser with no data sent to any server.