Emissive power of a body at a given temperature is the amount of energy emitted per unit time per unit area of the surface for a given wavelength.
The emissive power of a body is defined as the ratio of the amount of energy emitted (or radiated) per unit area per second to the amount of emitted heat energy per unit area of a perfectly black body per second at the same temperature.
The value of emissive power lies between 0 to 1. The emissive power of a perfectly black body is 1. But, a perfectly white surface or body has the zero emissive power.
Emissive power is denoted by eλ.
Its unit is W m-2.