The sun is the earth's primary source of energy. 

The sun, either directly or indirectly, is the ultimate source of almost all types of energy on Earth.

The sun directly provides us with solar power, which is an excellent non-conventional energy source.

Furthermore, fossil fuels (coal, oil, and gas) are transformed forms of plants (and animals) that once lived on the earth and grew by capturing the sun's energy.

Biomass is a byproduct of photosynthesis, in which the sun plays a key role.

Hydroelectricity is reliant on the water cycle, which is reliant on solar radiation.

Similarly, wind energy, tidal power, and wave power all rely on the sun in some way.