Energy transferred equation: E = Pt
Energy transferred = power × time. In symbols, E = P t. E is the energy in joules (J), P is the power in watts (W) and t is the time in seconds (s). AQA and both OCR courses print E = P t, while Edexcel gives the same link as P = E/t. Whichever board you sit, the equation sheet in the exam carries its version.
Equation
E = P t
| Symbol | Quantity | Unit |
|---|---|---|
E | energy transferred | J |
P | power | W |
t | time | s |
What each specification says
| Exam board | Status | Equation |
|---|---|---|
| AQA Physics (8463) | Recall | E = P t |
| AQA Combined Science (8464) | Recall | E = P t |
| Edexcel Physics (1PH0) | Not in this specification | |
| Edexcel Combined Science (1SC0) | Not in this specification | |
| OCR Gateway Physics A (J249) | Recall | E = P t |
| OCR Twenty First Century Physics B (J259) | Recall | E = P t |
Calculator: E = P t
What E = Pt tells you
Power is the number of joules transferred each second. If you know the power and how many seconds something runs, multiply them to find the total energy.
A 5 W phone charger left on for an hour transfers 5 J every second for 3600 seconds. That comes to 18 000 J. A 2000 W oven on for the same hour transfers 400 times as much.
How each board shows this link
- AQA (8463 and 8464): energy transferred = power × time, E = P t, on the recall list.
- OCR Gateway (J249) and OCR Twenty First Century (J259): E = P t, on the recall list.
- Edexcel (1PH0 and 1SC0): no E = P t line. Its recall list has power = energy transferred ÷ time taken, P = E/t. Its given list has energy transferred = current × potential difference × time, E = I V t.
Recall and given are the names each specification uses for its two lists. For papers sat in 2025, 2026 and 2027 all of them are printed in the exam, and the sheets carry on from 2028. Compare the Edexcel list with the OCR Gateway list to see the difference.
Rearranging and related forms
- P = E ÷ t, which has its own page as the power equation
- t = E ÷ P
For a circuit, the power is the p.d. times the current, so E = I V t. You can also find the energy from the charge that flows, using E = QV.
Worked examples
E
- Change the time to seconds: 2 × 60 × 60 = 7200 s
- E = P t
- E = 5 × 7200
- E = 36 000 J
36 000 J
t
- Change the energy to joules: 54 kJ = 54 000 J
- t = E ÷ P
- t = 54 000 ÷ 600
- t = 90 s
90 s
P
- Change the time to seconds: 3 × 60 = 180 s
- P = E ÷ t
- P = 9000 ÷ 180
- P = 50 W
50 W
Common mistakes
- Turning hours into seconds by multiplying by 60 once. An hour is 60 × 60 = 3600 seconds.
- Leaving the power in kilowatts. A 2 kW heater is 2000 W, and 2 in the equation gives an energy 1000 times too small.
- Giving the answer in watts. Power times time is energy, so the unit is the joule.
- Dividing when you should multiply. Energy grows the longer a device runs, so E gets bigger as t gets bigger.
Practise swapping E = Pt into t = E ÷ P in the Equation Rearranger.
Questions people ask
What is a kilowatt-hour?
It is the energy a 1 kW device transfers in one hour. Using E = P t, that is 1000 W × 3600 s = 3 600 000 J. Electricity bills count energy in kilowatt-hours because joules would give huge numbers.
Why does Edexcel not print E = Pt?
Its list has P = E/t instead, which is the same relationship. Multiply both sides by t and you have E = P t.