Electrical Power Calculator (P = VI)

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Find power, potential difference or current using P = V × I. Enter kilowatts or milliamps if the question uses them and the solver converts for you.

Quick answer

Power = potential difference × current. Use volts and amps to get watts. For example, a kettle on 230 V mains drawing 10 A has a power of 230 × 10 = 2300 W (2.3 kW).

By Mehmood Kakar · Updated

P = V × I
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How to calculate electrical power

The power of an electrical appliance depends on the potential difference across it and the current through it:

power = potential difference × current

  • P is power in watts (W)
  • V is potential difference in volts (V)
  • I is current in amperes (A)

UK mains electricity is 230 V. A kettle drawing 10 A from the mains has a power of 230 × 10 = 2300 W, or 2.3 kW.

Questions often give the power rating printed on an appliance and ask for the current. Rearrange to I = P ÷ V, and convert kilowatts to watts first.

If the question gives current and resistance instead of potential difference, use P = I²R. If it gives energy and time, use P = E ÷ t.

Common mistakes that cost marks

  1. Not converting kW to W. 2.3 kW is 2300 W. Multiply by 1000 before dividing.
  2. Not converting mA to A. 500 mA is 0.5 A.
  3. Using the wrong equation. P = VI needs potential difference and current. With current and resistance, use P = I²R.
  4. Dividing the wrong way. Current is P ÷ V. A kettle current of 0.1 A means you divided V by P.

Practice questions

Try each one on paper first, then reveal the worked answer.

Q1A kettle is connected to the 230 V mains supply. The current is 10 A. Calculate the power of the kettle.[2 marks]
  1. P = V × I
  2. P = 230 × 10
  3. P = 2300 W (2.3 kW)
Q2A 2.3 kW heater is connected to the 230 V mains supply. Calculate the current in the heater.[3 marks]
  1. Rearrange: I = P ÷ V
  2. Convert: P = 2.3 × 1000 = 2300 W
  3. I = 2300 ÷ 230
  4. I = 10 A
Q3A 6.0 W lamp has a current of 0.50 A through it. Calculate the potential difference across the lamp.[2 marks]
  1. Rearrange: V = P ÷ I
  2. V = 6.0 ÷ 0.50
  3. V = 12 V

Frequently asked questions

What is the equation for electrical power?

Power = potential difference × current (P = VI). Power is in watts (W), potential difference in volts (V) and current in amps (A).

What is the potential difference of UK mains electricity?

UK mains electricity is about 230 V at a frequency of 50 Hz.

How do I find the current from the power rating?

Rearrange to I = P ÷ V. A 2300 W appliance on 230 V mains draws 2300 ÷ 230 = 10 A.

When should I use P = I²R instead of P = VI?

Use P = I²R when you know current and resistance but not potential difference. Both give the same answer because V = IR.