Transformer power transfer

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 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:

potential difference across primary coil × current in primary coil = potential difference 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 in the primary coil (the input current).

(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 = \frac{power}{potential~difference~in~primary~coil}

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

Current = 0.174~A

The National Grid

When electricity is transmitted around the country on the National Grid, the transmission wires get hot and energy is dissipated to the surroundings.

Step-up transformers at a power station increase the voltage which reduces the current. The smaller the current, the less energy is dissipated so stepping-up the voltage makes the transmission of electricity more efficient.

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