$\text{Average Energy density of Electric field is } u_e = \frac{1}{2} \epsilon_0 E_0^2$

$\text{Average Energy density of Magnetic field is } u_m = \frac{B_0^2}{2\mu_0} $

$E_0^2  = c^2B_0^2 = \frac{B_0^2}{2\mu_0\epsilon_0}$

$\Rightarrow u_e = u_m$