The dimension of $\frac{1}{2}{\epsilon }_0{E}^2,$ where ${\epsilon }_0$ is permittivity of free space and E is electric field, is

Energy density of an electric field E is $u_E=\frac{1}{2}{\epsilon }_0{E}^2$ where ${\epsilon }_0$ is permittivity of free space $u_E=\frac{\text{ Energy }}{\text{ Volume }}=\frac{{\mathrm{ML}}^2{\mathrm{T}}^{-2}}{{\mathrm{L}}^3}={\mathrm{ML}}^{-1}{\mathrm{T}}^{-2}$

Hence, the dimension of $\frac{1}{2}{\epsilon }_0{E}^2\text{ is }{\mathrm{ML}}^{-1}{\mathrm{T}}^{-2}$