Power equation: P = E/t
Power = energy transferred ÷ time. In symbols, P = E/t. P is the power in watts (W), E is the energy transferred in joules (J) and t is the time in seconds (s). AQA, Edexcel and OCR Twenty First Century list this form. In your exam it sits on the printed equation sheet with the rest of your board's list.
Equation
P = E ÷ t
| Symbol | Quantity | Unit |
|---|---|---|
P | power | W |
E | energy transferred | J |
t | time | s |
What each specification says
| Exam board | Status | Equation |
|---|---|---|
| AQA Physics (8463) | Recall | P = E/t |
| AQA Combined Science (8464) | Recall | P = E/t |
| Edexcel Physics (1PH0) | Recall | P = E/t |
| Edexcel Combined Science (1SC0) | Recall | P = E/t |
| OCR Gateway Physics A (J249) | Not in this specification | |
| OCR Twenty First Century Physics B (J259) | Recall | P = E/t |
Calculator: P = E ÷ t
What power tells you
Power is how fast energy moves from one store to another. A 3000 W kettle and a 1000 W kettle can both boil the same water. The bigger one just does it in a third of the time.
One watt means one joule every second. So a 60 W lamp transfers 60 J each second it is switched on. When you see a power rating on an appliance, read it as joules per second.
Which boards list P = E/t
- AQA Physics (8463) and Combined Science (8464): P = E/t, on the recall list.
- Edexcel Physics (1PH0) and Combined Science (1SC0): P = E/t, printed twice, once in the energy topic and once in electricity.
- OCR Twenty First Century (J259): P = E/t, on the recall list.
- OCR Gateway (J249) does not list this form. It gives P = W/t and E = P t.
The recall list is each specification's own name for the equations it expects you to use with ease. For exams in 2025, 2026 and 2027, all the boards print that list on a sheet in every physics paper, and the sheets continue from 2028. The Edexcel page and the OCR Twenty First Century page show where this equation sits on each list.
Rearranging P = E/t
- E = P × t, to find the energy
- t = E ÷ P, to find the time
The first rearranged form has its own page: energy = power × time. If a question talks about work done by a force, the matching equation is power = work done ÷ time. Both give an answer in watts.
Worked examples
P
- Change the energy to joules: 180 kJ = 180 000 J
- Change the time to seconds: 1.5 × 60 = 90 s
- P = E ÷ t
- P = 180 000 ÷ 90
- P = 2000 W
2000 W
t
- t = E ÷ P
- t = 96 000 ÷ 800
- t = 120 s
120 s
Common mistakes
- Dividing by minutes. A heater that runs for 3 minutes has t = 180 s, and using 3 makes the power 60 times too big.
- Writing P = E × t. Power is a rate, so the energy is shared out over the time, which means you divide.
- Mixing kilowatts and watts in one calculation. Turn 2 kW into 2000 W before you use it with joules and seconds.
- Giving the answer in joules. Energy per second is measured in watts, so a number in J here is a sign you slipped.
Try turning P = E/t into t = E ÷ P against the clock in the Equation Rearranger.
Questions people ask
What is the difference between P = E/t and P = W/t?
W stands for work done, which is energy moved by a force. E covers any energy transfer, such as heating or light. The maths is the same and both give watts.
Does a higher power rating always mean more energy used?
Only if it runs for the same time. A 2000 W hair dryer used for one minute transfers less energy than a 100 W television left on for half an hour.
Why does Edexcel print it twice?
The same relationship appears in two topics of its specification, energy and electricity. It is one equation, so learn to spot it in both settings.