Watta

Meet Watta, a light bulb lifting weights to show how fast she can work.

Power is the rate at which work is done or energy is transferred: P = W/t. Two people can do the same job, but the one who finishes faster has more power. The SI unit is the watt, one joule per second, named after James Watt, whose steam engine improvements helped drive the Industrial Revolution. He also popularised the horsepower, now defined as about 745.7 W.

Lifting 10 kg by 2 m takes about 10 × 9.81 × 2 ≈ 196 J. Doing it in 2 s needs about 196 / 2 = 98 W; taking 4 s needs about 49 W. The same job at half the power takes twice as long.

Appliance labels give power so you can compare them: a kettle draws about two or three kilowatts, an LED bulb under ten watts. Electricity bills charge for energy, in kilowatt-hours, which is power multiplied by time.

P = W/t gives the average power over the time interval. Real machines also waste some energy as heat, so the power drawn is more than the useful power delivered.