Stars like the sun fuse 4 hydrogen nuclei together to form a single helium nucleus. The total mass of the 4 hydrogen nuclei is very slightly greater than the mass of the resulting helium nucleus, and that tiny "mass deficit" is converted into energy a la E = mc^2. A very tiny fraction of that energy reaches the earth mostly in the form of electromagnetic radiation - light, heat, radio, x-rays, ultraviolet, etc. - where plant life converts it into chemical energy by means of photosynthesis. ALL the earth's energy, except for that from radioactive decay - the spontaneous disintegration of very large atomic nuclei such as uranium - comes from the sun.