Power is the rate at which energy is transferred. It is measured in watts (W), and 1 W means 1 joule is transferred every second. A device with a higher power transfers more energy each second.
For any circuit device, the power depends on the potential difference across it and the current through it: power = potential difference × current, or P = VI. Power is in watts (W), potential difference in volts (V) and current in amperes (A). A kettle that draws 10 A from 230 V mains has a power of 230 × 10 = 2300 W.
Sometimes you know the current and the resistance but not the potential difference. Substituting V = IR into P = VI gives a second equation: power = current2 × resistance, or P = I2R, with resistance in ohms (Ω). A current of 2 A through a 5 Ω resistor gives P = 22 × 5 = 20 W. Because the current is squared, doubling the current for the same resistor makes the power four times bigger. This energy is transferred to the thermal store of the resistor and its surroundings.