Power

Power measures how quickly work is done, or how quickly energy is transferred from one store to another.

Key Definition The SI unit of power is the watt (W), where one watt equals one joule per second (1 W = 1 J s⁻¹).

Two equivalent equations define power. The first is:

P = W / t

where W is the work done in joules and t is the time in seconds. The second is:

P = ΔE / t

where ΔE is the energy transferred in joules. These two forms give the same numerical result because the work done on or by an object equals the energy transferred to or from that object during that process.

Both equations show that power is directly proportional to the work done (or energy transferred) when time is held constant, and inversely proportional to time when the work done (or energy transferred) is held constant. Doubling the work done in the same time doubles the power; completing the same work in half the time also doubles the power.

In calculations, all values must be in SI units before substitution — time in seconds, energy in joules, and power in watts.

Common conversions

• Minutes to seconds: multiply by 60
• Kilowatts (kW) to watts (W): multiply by 1000
• Kilojoules (kJ) to joules (J): multiply by 1000

Rearranging the equations algebraically allows the work done, energy transferred, or time to be calculated when the other two quantities are known.