Power is the rate at which energy is used or produced — energy divided by time. Confusing power with energy is the single most common mistake in this area: a 2,000 W kettle has a power rating, while the 0.1 kWh it uses to boil is energy. Your electricity bill charges for energy; the label on the appliance quotes power.
The watt is the SI unit, defined as one joule per second. Horsepower survives alongside it mostly through motor and vehicle specifications, and unhelpfully comes in several regional variants.
Conversion factors
| From | To | Multiply by |
|---|---|---|
| Kilowatts (kW) | Watts | 1,000 |
| Mechanical horsepower (hp) | Watts | 745.7 |
| Metric horsepower (PS) | Watts | 735.5 |
| BTU per hour | Watts | 0.2931 |
| Watts | Mechanical horsepower | 0.001341 |
| Kilowatts | Metric horsepower | 1.3596 |
Which horsepower is it?
Mechanical horsepower (745.7 W) is standard in the US and UK. Metric horsepower, written PS or CV, is 735.5 W and is what European car manufacturers usually quote. The 1.4% difference is small but real, and it explains why the same engine is advertised as 148 hp in one market and 150 PS in another. There is also boiler horsepower, an entirely different quantity at 9,810 W, which appears only in steam engineering.
Typical power draws
- LED bulb: 8–12 W, replacing a 60 W incandescent.
- Laptop: 30–90 W under load; a desktop with a discrete GPU, 300–600 W.
- Refrigerator: 100–200 W while the compressor runs, averaging far less.
- Electric kettle: 2,000–3,000 W — usually the largest draw in a kitchen.
- Domestic air conditioner: 1,000–3,500 W depending on size.
- Family car engine: 75–150 kW, or roughly 100–200 hp.
Turning power into cost
Multiply power in kilowatts by hours of use to get kilowatt-hours, then multiply by your tariff. A 2 kW heater running four hours a day uses 8 kWh, which at 25 cents per kWh costs $2 a day — around $60 a month. Working in kilowatts rather than watts from the start avoids a factor-of-a-thousand error, which is by far the most frequent mistake in energy budgeting.
Related tools
- Energy Converter — joules, kWh, calories and BTU
- Electricity Cost Calculator — appliance running costs
- Voltage Converter
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