In plane wave the H and E are in phase. So the pointing vector disappears regularly every pi. How is energy conservation validated ? Is it through the uncertainty of energy and time ?
Here is a paragraph on conservation of energy and maxwell's equations paragraph 1.6.
On average there is no problem. An instantaneous slice , where both E and B are zero, is misleading because they are changing in time in such a way as to build up the E and B of the next slice in t+dt.
In the framework where the plane wave is an ensemble of photons whose coherent stepping creates the E and B fields there is no problem of energy conservation even instantaneously, since the photons are not interacting and are each carrying their h*nu part of the energy of the wave. It is their synchronization in space time that creates the mathematical confluence of a zero in the classical formulation of the energy, that is why the classical discussion keeps to the average values of E and B..