Energy density is the amount of energy stored in a given system or region of space per unit volume, or per unit mass, depending on the context. In some cases it is obvious from context which quantity is most useful: for example, in rocketry, energy per unit mass is the most important parameter, but when studying pressurized gas or magnetohydrodynamics the energy per unit volume is more appropriate. In a few applications (comparing, for example, the effectiveness of hydrogen fuel to gasoline) both figures are appropriate and should be called out explicitly. (Hydrogen has a higher energy density per unit mass than does gasoline, but a much lower energy density per unit volume in most applications.)

Energy density per unit volume has the same physical units as pressure, and in many circumstances is an exact synonym: for example, the energy density of the magnetic field may be expressed as (and behaves as) a physical pressure, and the energy required to compress a gas may be determined by multiplying the pressure of the compressed gas times its change in volume.

In energy storage applications, the energy density relates the mass of an energy store to its stored energy. The higher the energy density, the more energy may be stored or transported for the same amount of mass. In the context of fuel selection, that energy density of a fuel is also called the specific energy of that fuel, though in general an engine using that fuel will yield less energy due to inefficiencies and thermodynamic considerations—hence the specific fuel consumption of an engine will be greater than the reciprocal of the specific energy of the fuel. And in general, specific energy and energy density are at odds due to charge screening.

Notes

• While all CO₂ gas output ratios are calculated to within a less than 1% margin of error (assuming total oxidation of the carbon content of fuel), ratios preceded by a Tilde (~) indicate a margin of error of up to (but no greater than) 9%. Ratios listed do not include emissions from fuel Cultivation/Mining, Purification/Refining & Transportation. Fuel availability is typically 74-84.3% NET from source Energy Balance.
• While Uranium-235 (²³⁵U) fission produces no CO₂ gas directly, the indirect fossil fuel burning processes of Mining, Milling, Refining, Moving & Radioactive waste disposal, etc. of intermediate to low-grade uranium ore concentrations, produces the equivalent CO₂ gas emissions per MJ of net-output-energy of a Natural Gas fired power station. Prof.Mark Diesendorf, Inst. of Environmental Studies, UNSW