in the field of electromagnetism, flux is usually the integral of a vector quantity over a finite surface. the result of this integration is a scalar quantity. the magnetic flux is thus the integral of the magnetic vector field over a surface, and the electric flux is defined similarly. using this definition, the flux of the poynting vector over a specified surface is the rate at which electromagnetic energy flows through that surface. confusingly, the poynting vector is sometimes called the "power flux", which is an example of the first usage of flux, above. it has units of watts per square metre (w/m2).