Electrical power is the energy transferred each second, and it is measured in watt (W). For any circuit device, power depends on the current in it and the potential difference across it: electrical power (W) = current (A) × potential difference (V), or P = I × V. A current of 3 A at 12 V gives a power of 3 × 12 = 36 W. If the current doubles and the potential difference stays the same, the power doubles.
Potential difference and current are linked to resistance by V = I × R. Replacing V in the first equation gives a second equation: electrical power (W) = current squared (A2) × resistance (Ω), or P = I2 × R. Current squared means current × current. A current of 3 A through a 10 Ω resistor gives 3 × 3 × 10 = 90 W, and 5 A through 4 Ω gives 5 × 5 × 4 = 100 W.
Use P = I × V when you know the current and the potential difference. Use P = I2 × R when you know the current and the resistance. A hair dryer that draws 8 A from a 230 V supply has a power of 8 × 230 = 1840 W.