Water waves are one half kinetic energy and one half potential energy. The quote below comes from the Wikipedia wave power article. I assume that it is the same with light waves. Do people here agree? "....E is the mean wave energy density per unit horizontal area (J/m2), the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy, both contributing half to the wave energy density E, as can be expected from the equipartition theorem."