When considering an electromagnetic plane wave, the electric and magnetic fields are maximum and minimum at the same time. What happens to the energy (Poynting vector S = E x B) as a function of time? It would seem that S is maximum when E and B are maximum and would be 0 when E and B are 0. The instantaneous energy at any given time seems to contradict energy conservation.

 

Thanks for your input.

When E and B are zero the energy is elsewhere in the wave — you have to look at an entire wavelength (or more, really; a true plane wave is infinite in extent)

You (the op) have a slight misunderstanding of what the Poynting vector is. S is a vector that describes the density of power (in watts /m^2) passing through a point and in which direction. Because light travels at a constant speed c, S is also a measure of energy density (joule/m^3) which as swansont implies can be volume integrated so you can "see" how energy moves and is conserved as it does so.