The most common primary energy sources we use today are fuels: petroleum products, natural gas products, biomass, and coal. Fuels are popular as energy sources because, as chemical energy, they are in a potential state. This means they can be stored for use when needed, unlike energy in a kinetic state.

This ability to transport and store fuels rather easily has spurred people to find ingenious ways to use them. Fuels are often used for moving vehicles and for heating or cooling. Fuels are also used as a primary energy source to produce electricity, a secondary energy source.

Advantages such as long-term storage and convenient transportation make fuels seem like the perfect energy sources for humankind. However, most fuels are not easily renewable. So, although kinetic energy sources are difficult to control over the short run (a cloudy day may block the sunlight you need), they are reliable in the long run (the sun comes up every day).