Efficiency of conversion is only a problem when there is a limited resource providing the original energy (although energy density is convenient for packaging).

The universe around us has nothing else in it BUT energy, fer goodness sake. The idea there is a scarcity of it is ridickle-dockle.
The winds, the tides, the shift of gravitic and magnetic fields, the sweeping solar storms, the ionosphere, the oscillations in every sound that rattles our streets and bodies as we go through our days, the heat from the sun and from the hot core of the earth, the fall of water down the planets gravity well, the boundless number of phtoons galloping through the atmospjhere... -- the list is practically endless. Not to mention the energy that ties the molecules around us into compounds and the nergy which bonds the nuclei of every atom to itself and the electrons around it to the nucleus.

A