Answer :

solar power is one, humans eat plants and plants make food by using the sun and the sun is a source of all energy
AL2006

The only widely-accepted activity of that sort is the universal habit
of operating our bodies on the solar energy stored in plants.

Other than that, the availability of free solar energy in the forms of
real-time wind and radiation is still not used to any great extent, as
a percentage of worldwide energy demand.  By and large, energy
consumers are still a lot more comfortable with the chemical energy
stored in wood, coal, and rotted dead dinosaurs.