Like other mobile games, there are limiters in place to prevent players from spending hours and hours playing the game. One such system is the Stamina system, which is also used by other popular mobile games out there.
Energy is primarily used for tackling story stages at 3 stamina per stage. Even if you’re using the Express mode, stamina will still be required per Mission Express ticket used. Thankfully, energy seems to be usable only in the story stages as the other instances like dungeons and towers have their respective keys.
Energy naturally recovers at a rate of 1 energy every 10 minutes. This rate can’t be changed or sped up. Once max energy is achieved, the natural energy recovery will be deactivated until energy is used again. It is possible to go over the max energy capacity by opening energy chests.
You can choose to recover some energy by opening Campaign Energy chests that you’ll get as rewards or buy them from the shop. The game also regularly sends a Meal Energy Gift via in-game mail that replenishes 15 energy. Don’t forget to claim these whenever you’re logged in.
We have questions and answers related to this topic which may also help you: