Definition · Plain-language
Power in physics
In physics, power is the rate at which work is done or energy is transferred — how quickly energy is used.
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The rate of doing work
Power answers the question "how fast?" rather than "how much?". It is the rate at which work is done or energy is transferred, calculated as the energy transferred divided by the time taken. Two cranes might lift the same load to the same height — doing the same work — but the one that does it in half the time has twice the power. Power is a scalar quantity, and its SI unit is the watt, defined as one joule of energy transferred each second.
Watts, kilowatts and horsepower
Because the watt is a small unit, larger amounts are given in kilowatts (a thousand watts) or megawatts (a million). A household kettle might draw two to three kilowatts, while a power station produces hundreds of megawatts. The older unit horsepower, still used for engines, equals about 746 watts. The familiar kilowatt-hour on an electricity bill is not a unit of power at all but of energy — the energy used by a one-kilowatt device running for one hour — a common source of confusion.
Power, force and speed
For something moving, power can also be expressed as force multiplied by velocity. This is why a car needs more engine power to maintain a high speed against air resistance, and why climbing stairs quickly requires more power than strolling up them, even though the work done against gravity is the same. Understanding power matters in everyday life: appliance ratings, engine performance and energy efficiency are all about how quickly energy is delivered or consumed.
Key facts
At a glance
- Definition: the rate at which work is done or energy is transferred
- Formula: power = energy transferred ÷ time taken
- SI unit: the watt (W), equal to one joule per second
- Quantity type: a scalar — magnitude only
- Larger units: kilowatt (1,000 W), megawatt (1,000,000 W)
- Not energy: a kilowatt-hour measures energy, not power
Common misconceptions
What people often get wrong
Often heard: Power and energy are the same thing.
Actually: Energy is the total capacity to do work, in joules; power is how fast that energy is used, in watts. A low-power device left on for a long time can use more energy than a high-power one used briefly.
Often heard: A kilowatt-hour is a unit of power.
Actually: A kilowatt-hour is a unit of energy — the energy a one-kilowatt device uses in one hour. Power is the rate of energy use, measured in watts, not in kilowatt-hours.
Often heard: More powerful always means more total work.
Actually: Greater power means work is done faster, not necessarily that more work is done. A powerful and a weak motor can do the same total work; the powerful one simply finishes sooner.
Going deeper
