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
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.