As far as I know, a GL never reached his limit in how much energy they can have. Considering the 24 hopur charge, ive never heard of them not doing something becuase they donr have enough energy besides needing a recharge. You say OA energy is not equal to the power of the universe, but OA energy/willpower is gathered from every living being in the universe. Then all that energy gets amplified by the wearer a million times over. Thats why containg something such as a supernova wouldnt be so hard to believe.