Relationship between power and energy density

Relation between density of states and energy

Energy is usually measured in Joules, the other units include ergs and calories. Power is measured in Watts, which is Joules per second, or ergs per second. Machine are usually described by its power rating, the higher the number the more powerful the machine.

Power is an energy per unit of time. As a rate of change of work done or the energy of a subsystem, power is:
P = W/t, where P is power, W is work and t is time.

The capacity of a man that allows him to do work is called energy. In simple words, energy is the ability to work. It is in scalar quantity and has only magnitude and no direction. energy can neither be created nor destroyed, it can only change in its form.