$ε = ∫Pdt$
∵For a plane wave, S is constant over the area, so:
$P =SA=(EH)A=(E(ϵcE))A=(ϵcE²)A \\ ∴ε = ∫Pdt = ∫(ϵcE²)A·dt$
$∴ε = ∫(ϵcE²)A·dt = (ϵE²) ∫A·c·dt$
∵·c·dt is the length and A is the area
∫A·c·dt gives a volume
$∴ε = ϵE²·V$