Energy Density is the amount of energy stored in a given (unit) volume, e.g., J/m³ (Joules per meter cubed). The concept has a lot of use in engineering (e.g., batteries) and science, including astrophysics: often the calculation of the energy in a system provides a way to model its behavior. One use is in Radiative Transfer models, such as using the density of energy of the Electromagnetic Radiation passing through a volume.