Nuclear reactors use light as the energy for power.

Coal generators use light as the energy for power.

Natural gas generators use light as the energy for power.

Geothermal generators use light as the energy for power.

Light is the way energy is transferred from one atom or molecule to another atom or molecule.

Heat is light.

For example:

Coal is burned, the chemical reaction creates lots of photons. Those photons get absorbed by water molecules. The water molecules become excited. Excited water molecules move faster. Faster moving water molecules have more kinetic energy. Steam is the result. High velocity steam spins a turbine. The turbine spins a magnetic field inside an coil. This produces electricity.

Energy density is really a measure of the number and energy of the photons generated by a certain fuel. Light is not the fuel (hint: fusion).