Can someone explain how I can bank energy and NOT lose it overnight?

If you have the original energy bar, it will autofill 1 energy every 2 minutes until it reaches the maximum of 110 energy (takes 3.7 hours). If you bought the Increased Capacity Bar with 799 diamonds, then your maximum energy stored is 150 energy (takes 5 hours). You won't lose energy overnight, you just won't gain any more than the max over time. However, you can stockpile energy beyond the maximum by having a full bar before accepting your teammate's energy drinks, completing the photo album's jigsaw puzzles, or activating a starbooster.