A source of energy is one which can provide appropriate amount of energy in expedient form over a long period of time.

Two main categories of the source of energy are:

a) Renewable source of energy-source of energy that is not depleted when used. Such as wind or solar power.

b) Non- renewable source of energy-sources of energy that will run out or will not be replenished in our lifetimes or even in many, many lifetimes.