$\overrightarrow{S}(t)=\overrightarrow{E}(t)\times \overrightarrow{H}(t)$

of the electric field vector $\overrightarrow{E}(t)$ and the magnetic field vector $\overrightarrow{H}(t)$ at a point in an electromagnetic field

NOTE 1 – The Poynting vector flux through a closed surface is equal to the power passing through this surface.

NOTE 2 – For a periodic electromagnetic field, the time average of the Poynting vector is a vector of which, with certain reservations, the direction may be considered as being the direction of propagation of energy and the magnitude considered as being the average power flux density.