The energy associated with the electric field is (UE) and with the magnetic field is (UB) for an electromagnetic wave in free space. Then:

Average energy density of magnetic field, uB = B²₀/2μ₀, B₀ is the maximum value of the magnetic field.
Average energy density of electric field, uE = ε₀E²₀/2
Now, E₀ = CB₀, C² = 1/μ₀ε₀
uE = E₀²/2C²B₀² = E₀²/2(1/μ₀ε₀)B₀² = B₀²/2μ₀ = uB
Since the energy density of the electric and magnetic fields is the same, the energy associated with an equal volume will be equal.
uE = uB