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Power calculator
Energy and power
Power is the rate of transferring energy, or the work done per second.
P = E / t
This free calculator is for GCSE and A-level physics. Choose the quantity you want to find, enter the other values with their units, and it shows the working in the layout used in exams, with the answer to the number of significant figures you choose. The values shown are an example. Change them to your own.
Quantity to find
Working
- Write the equationP = E / t
- Substitute the valuesP = 7200 ÷ 60
- CalculateP = 120 W
- Give the answer to 3 significant figuresP = 120 W
Worked example
Find P, the power. Given: energy transferred or work done E = 7200 J; time t = 60.0 s.
- Write the equationP = E / t
- Substitute the valuesP = 7200 ÷ 60
- CalculateP = 120 W
- Give the answer to 3 significant figuresP = 120 W
Symbols and units
| Symbol | Quantity | SI unit |
|---|---|---|
| P | Power | W |
| E | Energy transferred or work done | J |
| t | Time | s |
Power: rearranged
- To find power: P = E / t
- To find energy transferred or work done: E = P t
- To find time: t = E / P
Exam tip
One watt is one joule per second. Convert minutes and hours to seconds, and kJ to J.
Common questions
What is the formula for power?
The equation is P = E / t, where P is the power (W); E is the energy transferred or work done (J); t is the time (s).
What units should I use for power?
In SI units: power in W, energy transferred or work done in J, time in s. The calculator also accepts common alternatives such as kilometres, grams or minutes, and converts them for you.
How do I rearrange P = E / t?
Rearranging gives these forms: to find power: P = E / t. to find energy transferred or work done: E = P t. to find time: t = E / P.
Practise power questions
Press the button for a question with random numbers. Work it out on paper first, then type your answer.
Related
Read the full explanation on the Power from energy and time formula page. More energy and power equations: Work done, Work done by an angled force, Elastic potential energy, Power from force and speed, Kinetic energy, Gravitational potential energy, Energy transferred, Efficiency. See the full list on the physics equation calculator page.