Explain that ultimately we have to depend upon solar energy.

Exploitation of conventional sources of energy is don to perform any type of work. Some of the conventional sources of energy like coal and petroleum is found beneath the surface of the earth. The rate at which these sources are consumed shows that they will be exhausted within a few years. Another source of energy, nuclear energy through nuclear fission and fusion reaction is also exhaustible. 
Ultimately, the mankind will rely on solar energy which is available in plenty in our universe. Hence, it is called as the ultimate source of energy.