The problem statement, all variables and given/known data A parallel plate capacitor is air filled and assumed to be ideal. It is connected to a battery so it has constant potential difference. By what factor does stored energy change if the charge is doubled.. The attempt at a solution Using the formula U = 0.5QV.. If charge is doubled, then U is doubled correct? I say the answer is stored energy is doubled, but the answer at the back reckons quadrupled. Who is right?