Energy is the potential to do work. But work is done only when the energy is released.
A disconnected battery isn't doing work, but it has the potential to do work, so it's a source of energy.
The difference in electron supply at the battery terminals is sometimes called the potential difference - in the case of a standard charged automotive battery, a potential of 12 volts. Tapping this potential means turning one form of energy, the battery's electrochemical energy, into another.
Turning one form of energy into another is called work.
The amount of energy transformed is the amount of work done. When a person's legs turn the pedals of a bicycle, physical energy is being turned into mechanical energy. A power drill turns electrical energy into mechanical energy.
In each case, work is being done, but the power drill is different – it does the work quicker, delivering more mechanical energy. That difference is called Power. Power is the rate at which work is done. The rate of transforming energy. In an electrical circuit, power refers to the rate at which electrical energy is transformed into another kind of energy
The unit of power is the watt. 1 watt is produced when 1 volt causes a current flow of 1 amp. From this comes the power equation: P, the power in watts, equals V, the voltage in volts, multiplied by I, the current in amps. This calculation is applied just like Ohm's law.
When current flows in a circuit with a resistor in it, the resistor may become hot as it converts electrical energy into heat energy. If this circuit is powered by a 12 volt battery with a current of 2 amps, using the power equation (P=VxI), we can determine that 24 watts of power are being taken from the circuit by the resistor
It is also possible to simplify and transpose the power equation:
