EM wave transports energy in space, which is contained in electromagnetic field. The intensity and velocity of wave are computed with the wave equation, which will be used to derive the energy flux of wave. The flux of energy is the quantity of energy carried by a wave that crosses a surface. Energy flux must respect the energy conservation law. However, this is not the case for our EM wave equation as shown in the following.

Please read the article at
http://pengkuanem.blogspot.com/2012/12/electromagnetic-wave-energy-flux.html
or http://www.academia.edu/2313022/Electromagnetic_wave_energy_flux