U = \frac{1}{2}$$CV^2 = $\frac{1}{2}$QV = Q^$\frac{2}{2C}$. All three forms are equivalent (from C = $\frac{Q}{V}$). The factor 1/2 arises because the average voltage during charging is V/2. The energy is stored in the electric field between the plates, not on the plates themselves. Energy density (per unit volume): u = $\frac{1}{2}$$epsilon_0$$E^2$ (in vacuum) or u = $\frac{1}{2}$K$epsilon_0$$E^2$ (in dielectric). Total energy = integral of udV over the volume where E exists. For parallel plates: U = $\frac{1}{2}$$epsilon_0$$E^2$(Ad) = $\frac{1}{2}$$\frac{epsilon_0*A}{d}$$V^2$ = \frac{1}{2}$$CV^2 (consistent). The concept of energy density is powerful for JEE — it shows energy is localized where the field exists.