Transformer power transfer

To calculate electrical power use the equation:

power = potential difference × current

P = V I

This is when:

  • power (P) is measured in watts (W)
  • potential difference (V) is measured in volts (V)
  • current (I) is measured in amperes – also referred to as amps – (A)

Assuming that a transformer is 100% efficient, the following equation can be used to calculate the power output from the transformer:

potential difference across primary coil × current in primary coil = potential difference across secondary coil × current in secondary coil

V_s \times I_s = V_p \times I_p


A step-down transformer converts 11 500 V into 230 V. The power output is used to run a 2,000 W kettle. Calculate the current flowing in the primary coil.

From P = V I, kettle~power = V_s \times I_s = 2,000~W

V_p \times I_p = V_s \times I_s

So V_p \times I_p = 2,000~W

I_p = 2,000 \div 11,500

input current, I_p = 0.174~A

High voltage power transmission

The National Grid carries electricity around Britain. The higher the current in a cable, the greater the energy transferred to the surroundings by heating. This means that high currents waste more energy than low currents.

To reduce energy transfers to the environment, the National Grid uses step-up transformers to increase the voltage from power stations to thousands of volts, which lowers the current in the transmission cables. Step-down transformers are then used to decrease the voltage from the transmission cables, so it is safer to distribute to homes and factories.

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