1 Watt (W) = 1 Joule per second (J/s). The Watt is the SI unit of power — the rate at which energy is transferred or work is done. Named after Scottish engineer James Watt.
James Watt invented the separate condenser for the steam engine, dramatically improving its efficiency. He used the term 'horsepower' as a marketing tool to convince mine owners that his engines could replace their horses — and it stuck.
Power = Work done / Time taken P = W/t
When 1 joule of work is done in 1 second: 1 Watt = 1 Joule / 1 second = 1 J/s
In base SI units:
Dimensional Formula: [ML²T⁻³]
| Equivalent | Value |
|---|---|
| 1 W = | 1 J/s |
| 1 kW = | 1000 W |
| 1 MW = | 10⁶ W |
| 1 horsepower (hp) = | 746 W |
| 1 W = | 0.00134 horsepower |
| 1 kWh = | 3.6 × 10⁶ J (= 1 unit of electricity) |
1 horsepower = 746 W is a key exam fact — James Watt defined horsepower to compare his steam engines to horses.
| Device | Power |
|---|---|
| LED bulb | 8–12 W |
| CFL bulb | 15–25 W |
| Ceiling fan | 75 W |
| Desktop computer | 200–400 W |
| Electric kettle | 1500–2000 W |
| Air conditioner (1 ton) | ~1000 W |
| A human body (at rest) | ~80 W |
Your electricity bill is calculated in kilowatt-hours (kWh) — also called a unit:
1 unit = 1 kWh = 1000 W × 3600 s = 3,600,000 J
If a 100W bulb runs for 10 hours: Energy consumed = 100 × 10 = 1000 Wh = 1 kWh = 1 unit of electricity
**Watt (W)** measures power (rate of energy use). **Watt-hour (Wh)** measures energy (total energy used over time). A 60W bulb running for 1 hour uses 60 Wh of energy. Power is instantaneous; energy accumulates over time.
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