The Dinorwig pumped storage hydroelectric facility in Northern Wales utilizes an abandoned slate quarry named Marchlyn Mawr as the upper reservoir. 600 meters below the Llyn-Peris lower reservoir can be found. The plant contains six 288MW generators. Given the overall facility efficiency is 76%, and the plant is capable of full output for a duration of 5 hours, determine the potential energy of upper reservoir and the minimum amount of water in gallons it must hold.