Power rating is the maximum power an appliance can safely operate at. This means the power rating of an appliance tells us the maximum energy transferred per second by an appliance. However, having a higher power rating does not mean that more useful energy is transferred. A high power rating appliance may not be as efficient as a low power rated appliance.

Energy transferred can also be calculated by working out how much work has been done by the charge carriers that have flowed through the circuit across a potential difference:

• $E$= energy in joules (J)
• $Q$= charge in coulombs (C)
• $V$= potential difference in volts (V).

Recall that power can be calculated using the equation:

• $P$= power in watts (W)
• $V$= potential difference in volts (V)
• $I$= current in amps (A).

If you don't know the potential difference, you can also calculate power using:

• $P$= power in watts (W)
• $I$= current in amps (A)
• $R$= resistance in ohms (Ω)