# Does energy of a capacitor means energy stored in both plates?

I've a doubt in this,

Does the term potential energy of a parallel plate capacitor means the energy stored in both the plates or a single plate, since the formula $$E=Q^2/2C$$ , $$Q$$ is the charge of only one plate?

It is the energy for the entire system. It is the energy required to put $$+Q$$ on one plate and $$-Q$$ on the other plate.
Consider that you have an uncharged parallel plate capacitor and you decide to charge it up such that the charge on one plate is $$+Q$$ and the charge on the other plate is $$-Q$$ where $$Q>0$$. At any point in time, the charge on the positive plate is $$q$$, where $$0\leq q\leq Q$$, which gives a potential difference between the plates of $$\frac{q}{C}$$.
We know that $$dW=Vdq$$ and so $$W=\frac{1}{C}\int_0^Qqdq=\frac{Q^2}{2C}$$ As you can see, the derivation of the formula above utilized the potential difference between both plates and so the state of the second plate was also taken into account. Therefore, $$\frac{Q^2}{2C}$$ is the total energy of the entire system.