The two plates X and Y of a parallel plate capacitor of capacitance C are given a charge of amount Q each. X is now joined to the positive terminal and Y to the negative terminal of a cell of emf E = Q/C.

As the emf of cell is ϵ=Q/C, charge of amount Q will flow from the positive terminal to the negative terminal of the cell through the capacitor.
As the Y is connected to negative terminal of cell so it is grounded and the total charge on plate Y will be zero. 
Here cell charge the capacitor plate X and its total charge =(Q+Q)=2Q
The energy will supply by cell is ${\mathrm{CE}}^2$