By definition, a capacitor is a device that stores energy in the form of an electric field. When a capacitor is connected across a voltage source electrons will flow from the negative terminal of the battery to the plate of the capacitor that is connected to the said terminal hence the plate will be negatively charged.
Also, the electrons will be attracted from the other plate of the capacitor to the positive terminal of the battery or the negatively charged plate will attract the positive charge on the other plate and the plate will become positively charged. This process will continue until the charge on the negative plate begins to repel the further accumulation of electrons on the plate. Now the capacitor is fully charged.
Note here only a charge imbalance is created on the plates of the capacitor, considering the capacitor as a system no charge is added or subtracted to the capacitor. If we connect positive and negative terminals of the battery this charge will be neutralized. The capacitor is a device that only holds the imbalance of charge. Due to this charge imbalance, an electric field is created between the two oppositely charged plates of the capacitor which in fact is the reason due to which capacitors can store energy.
The maximum charge that can accumulate is proportional to the applied voltage and capacitance of the capacitor.
Q = CV
Q = Charge
C = Capacitance
V = Volts
Since capacitance depends on the physical parameters of the capacitor, the charge imbalance that can be created also depends on the physical parameters of the capacitor.
It is further added that the capacitors store charge is a misconception. However, it seems convenient to think that the capacitor is filling with charge and there is no harm in thinking that way until unless it is for circuit analysis point of view.
As mentioned earlier a charge imbalance is created on the capacitors by applying the voltage across the capacitor. Work done is required to create this charge imbalance. This work is what is actually translated into energy that is actually stored in the capacitor. The amount of energy that is stored in the capacitor can be represented mathematically using the following equation [4-73]
E = ½ CV2
E = Energy
C = Capacitance
V = Voltage