Equation shown in the image is the Stefan-Boltzmann law. “P” is the power radiated by the ideal radiator and ‘σ‘ is is the Stefan-Boltzmann constant, which has the value

σ = 5.67*10^-8 Wm^-2K^-4

Where ‘P’ is in Watts, emissivity (e) has no units, Area (A) m² and Temperature (T) is in Kelvin. In the thermodynamic analysis the energy density is related from a square per unit time and per unit area. Thus the heat interior surface of an enclosure produces a certain energy density of thermal radiation in the enclosure. We are interested in radiant exchange with surfaces – hence the reason for the expression of radiation from a surface in terms of its temperature.