Power is the rate at which energy is transferred by a device. It is measured in watts (W). A power of 1 watt means 1 joule of energy is transferred every second. The power of any circuit device depends on the potential difference across it and the current through it.
The first equation is power = potential difference × current, or P = V I. Power is in watts (W), potential difference is in volts (V) and current is in amperes (A). A lamp with 12 V across it and a current of 3 A has a power of 12 × 3 = 36 W.
The second equation is power = current2 × resistance, or P = I2R, with resistance in ohms (Ω). It comes from combining P = V I with V = I R, and it is useful when you know the current and the resistance but not the potential difference. A 5 Ω resistor with a current of 2 A has a power of 22 × 5 = 4 × 5 = 20 W, and this energy is transferred as heat.
Because the current is squared, doubling the current through a resistor makes the power four times bigger. Power is squared current times resistance, so current matters most.