Energy conversion efficiency - Wikipedia
https://en.wikipedia.org/wiki/Energy_conversion_efficiencyEnergy conversion efficiency depends on the usefulness of the output. All or part of the heat produced from burning a fuel may become rejected waste heat if, for example, work is the desired output from a thermodynamic cycle. Energy converter is an example of an energy transformation. For example, a light bulb falls into the categories energy converter. Even though the definition includes the notion of usefulness, efficiency is considered a technical or physical term. Goal or mis…
Energy density - Wikipedia
https://en.wikipedia.org/wiki/Energy_densityIn physics, energy density is the amount of energy stored in a given system or region of space per unit volume. It is sometimes confused with energy per unit mass which is properly called specific energy or gravimetric energy density. Often only the useful or extractable energy is measured, which is to say that inaccessible energy (such as rest mass energy) is ignored. In cosmological and other general relativistic contexts, howe…