We now know that alternative energy sources are any source we use to supplement or even replace traditional energy sources used for power generation. You could almost say the same thing about renewable energy sources. But there is one subtle difference between the two. All renewable energy sources fall under the category of alternative energy sources, but it doesn’t work the other way around. That’s because renewable energy sources are derived from naturally replenished sources or processes of Earth, such as the sun, wind, and water. We refer to these resources as renewable or sustainable (as in sustainable energy) since, unlike fossil fuels, this naturally occurring continual renewal makes them inexhaustible. However, it’s possible for there to be alternative energy sources that are exhaustible, and therefore not renewable.