# Power

How fast energy is transferred. Power = energy / time, measured in watts (W). One watt is one joule per second.

Physics (school) · Part 03: Energy, work and power · Card 6 of 8 · Last updated 2026-10-19

## Analogy

Two people climb the same stairs. The quicker one has more power, because the energy moved is the same.

## Example

**Kettle**

energy = 360,000 J
time = 180 s
power = 360,000 / 180
power = 2,000 W

## In practice

A 2,000 W kettle moves 2,000 J of electrical energy into the water every second.

## Practice

### Flashcards

- **What is power and what is its unit?** Power is energy transferred per second. Its unit is the watt (W).

### Interview questions

**A 60 kg student climbs stairs 3 m high in 6 s. Take g = 10 N/kg. Calculate the work done and the power.**

- Gain in GPE = 60 x 10 x 3 = 1,800 J
- Work done equals the energy transferred, 1,800 J
- Power = 1,800 / 6 = 300 W

## Questions people ask

### What is the difference between energy and power?

Energy is how much is transferred, in joules. Power is how fast it is transferred, in watts. A 60 W bulb and a 100 W bulb can both run all day, but the 100 W bulb transfers energy faster and so uses more energy in the same time.

### How do I calculate power from work done?

Divide work done by the time it took: P = W / t. If you do 600 J of work lifting a box in 3 seconds, your power is 200 W. Make sure the time is in seconds and the work is in joules before you divide.

## Related concepts

- [Gravitational potential energy](https://studyvisually.com/learn/science/physics-school/physics-school-part-3/physics-school-part-3-concept-4)
- [Work done](https://studyvisually.com/learn/science/physics-school/physics-school-part-3/physics-school-part-3-concept-5)
- [Energy stores and transfers](https://studyvisually.com/learn/science/physics-school/physics-school-part-3/physics-school-part-3-concept-1)
- [Conservation of energy](https://studyvisually.com/learn/science/physics-school/physics-school-part-3/physics-school-part-3-concept-2)

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