Transformer power transfer - Higher

In an electrical circuit, energy is transferred each second. This is electrical power. The power output of a transformer can be calculated using the equation:

power = potential difference × current

This is when:

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

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

pd across primary coil × current in primary coil = pd across secondary coil × current in secondary coil


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 (the input current).

Note, the power in each coil will be the same, since transformers are 100 per cent efficient.

Power = 2,000 W

Rearrange the formula to find current:

Current in primary coil = power ÷ potential difference in primary coil

current~in~primary~coil = \frac{2,000}{11,500}

current = 0.174~A

Electrical working in the National Grid

The equation above only works if energy cannot be dissipated through heat, or other ways. In reality, as energy is transferred, the core and wires in the transformer will heat up due to induced currents. The amount of heating depends on the resistance of the wires and how big the current is.