$\frac{{\mathrm{E}}^2}{{\mathrm{\mu }}_0}$ has the dimensions (E= electric field, ${\mathrm{\mu }}_0$ = permeability of free space)

$\begin{array}{l}\left[\frac{{\mathrm{E}}^2}{{\mathrm{\mu }}_0}\right]=\left[\frac{{\mathrm{\epsilon }}_0{\mathrm{E}}^2}{{\mathrm{\epsilon }}_0{\mathrm{\mu }}_0}\right]=\left[\frac{\text{ energy }/\text{ volume }}{(1/\text{ speed of light }{)}^2}\right]\\ \Rightarrow \left[\frac{{\mathrm{E}}^2}{{\mathrm{\mu }}_0}\right]=\left[\frac{\text{ energy }(\text{ speed }{)}^2}{\text{ volume }}\right]\\ \Rightarrow \left[\frac{{\mathrm{E}}^2}{{\mathrm{\mu }}_0}\right]=\left[\frac{{\mathrm{ML}}^2{\mathrm{T}}^{-2}{\mathrm{L}}^2{\mathrm{T}}^{-2}}{{\mathrm{L}}^3}\right]=\left[{\mathrm{MLT}}^{-4}\right]\end{array}$