Electrical power equation: P = VI
Power = potential difference × current. AQA and OCR print it as P = V I, and Edexcel prints P = I V. P is the power in watts (W), V is the potential difference in volts (V) and I is the current in amperes (A). Each board's version is printed on the sheet you get with your exam paper.
Equation
P = I V
| Symbol | Quantity | Unit |
|---|---|---|
P | power | W |
I | current | A |
V | potential difference | V |
What each specification says
| Exam board | Status | Equation |
|---|---|---|
| AQA Physics (8463) | Recall | P = V I |
| AQA Combined Science (8464) | Recall | P = V I |
| Edexcel Physics (1PH0) | Recall | P = I V |
| Edexcel Combined Science (1SC0) | Recall | P = I V |
| OCR Gateway Physics A (J249) | Recall | P = V I |
| OCR Twenty First Century Physics B (J259) | Recall | P = V I |
Calculator: P = I V
What the equation says
Every appliance you plug in turns electrical energy into something else: heat in a toaster, motion in a fan, light and sound in a TV. The power is how many joules it transfers each second.
Two things set that rate. The p.d. tells you how much energy each coulomb of charge carries. The current tells you how many coulombs pass each second. Multiply them and you get joules per second, which is watts.
Power ratings are printed on a small plate on most appliances. A phone charger might say 10 W, a hair dryer 2000 W. Those numbers let you work out the current each one draws from a 230 V socket.
Board by board
- AQA (8463 and 8464): power = potential difference × current, P = V I.
- Edexcel (1PH0 and 1SC0): electrical power = current × potential difference, P = I V.
- OCR Gateway (J249) and OCR Twenty First Century (J259): P = V I.
The order of the letters changes, and the answer does not. Every one of these is on its specification's recall list. For each exam series from 2025, the board prints that list in the paper. The Edexcel equation list shows its P = I V beside the other electricity equations.
Rearranging and related equations
- I = P ÷ V
- V = P ÷ I
If you know the resistance but not the p.d., use P = I²R instead. It comes from putting V = IR into P = V I. To find the total energy an appliance uses, multiply the power by the time with E = Pt.
Worked examples
P
- P = V I
- P = 230 × 4
- P = 920 W
920 W
I
- Change the power to watts: 3 kW = 3000 W
- I = P ÷ V
- I = 3000 ÷ 230
- I = 13.0 A (to 3 significant figures)
13.0 A
V
- V = P ÷ I
- V = 10 ÷ 2
- V = 5 V
5 V
Common mistakes
- Dividing 3 by 230 for a 3 kW kettle. The power must be in watts first, or the current comes out as a tiny 0.013 A.
- Mixing up the unit W and the letter W. On this page W is the watt, the unit of power, and has nothing to do with weight.
- Using the p.d. of the mains when a question gives a battery. Read the value from the question each time.
- Rounding the current too early. Keep the full value on your calculator and round only the final answer.
Rearrange P = VI for current or p.d. in the Equation Rearranger.
Questions people ask
Why does Edexcel write P = IV and AQA write P = VI?
Multiplication works in any order, so both are right. Use the one your board prints, so it matches the sheet in your exam.
How does this help choose a fuse?
Work out the normal current with I = P ÷ V. The fuse rating should be a little above that current, so it melts only when too much current flows.