The energy stored in a capacitor is: $E = \frac{1}{2}CV^2 = \frac{Q^2}{2C} = \frac{1}{2}QV$. The most common expression is $E = \frac{1}{2}CV^2$, where C is capacitance (in Farads) and V is voltage. All three forms are equivalent and correct expressions for stored electrostatic energy.