Electric field energy density ($u_E={ϵ}_0|E{|}^2/2$)

The local energy per unit volume associated with an electric field in vacuum is $u_E={ϵ}_0|E{|}^2/2$. Its volume integral gives the electric contribution to the electromagnetic energy. For a static field outside an isolated charged conducting sphere, integrating this density from its radius to infinity gives the electrostatic energy $Q^2/(8\pi {ϵ}_{0}R)$.