The power of a device is the energy it transfers each second. It is measured in watts (W), where 1 W is 1 joule per second. The power depends on both the potential difference across the device and the current through it: power = potential difference × current (P = IV).
Why? The potential difference is the energy transferred for each coulomb of charge that passes through the device (E = QV). The current is the charge passing each second (Q = It). A bigger potential difference means each coulomb transfers more energy. A bigger current means more coulombs pass each second. Either way, energy is transferred faster and the power is higher.
The energy transferred in a given time is energy = power × time (E = Pt). For example, a kettle on 230 V with a current of 10 A has a power of 2300 W. In 60 s it transfers 138000 J. Using P = I2R also gives the power when you know the current and resistance.