There are three ways to get more energy. It will regenerate at the rate of 1 every 5 minutes (3 minutes if you are a Maniac). It will completely refill whenever you gain a new level. This is one of the principal facts that makes speed leveling work. You can also get a full refill from the Godfather. Finally there are Energy Paks and there can be a lot of confusion over how these work when you first start playing.
Energy Paks will completely refill your energy to your current maximum and give you an extra 25%. Your energy won’t refill again until your amount of current energy drops below your maximum. If you level before you use the extra 25% energy, you will lose the extra and be reset to your maximum amount. You want to save your Energy Pak until you have used all of your energy and you have the ability to consume most of the energy before leveling. On a similar note, you want to use the highest XP payout job if it is about to cause you to level as any excess XP beyond what is necessary to level will spill over in to starting the XP needed for the next level.
