From Dragon Mania Legends (DML) Wiki
Portal Energy is the energy used each time a battle is fought on the Campaign Map for the Normal Mode, Heroic Mode, and Nightmare Mode campaigns. Portal Energy regenerates automatically at 1 unit every 25 minutes, up to the maximum base amount. It can also be purchased with Gems (at the cost of 3 per energy), received when leveling up, or refilled when upgrading the Ancient Portal. Those methods fully replenish the energy; there is no option to select the amount of energy to replenish. The only way to replenish smaller amounts of energy is to use small amounts at a time, and then replenish the amount remaining to maximum. Portal Energy is typically refilled by pressing the + button while on the Campaign Map, or by trying to engage a battle without any energy left (which opens the refill screen, where energy can be refilled using Gems). Energy can also be received as a gift from visiting players, from events, or by using the related Clan Fortress Power if the Trainer is a member of a Clan.
A player's base amount of energy is determined by their Ancient Portal level, the initial amount being three, while the maximum is nine. Trainers with an active VIP Status receive additional energy to their base amount. VIP levels 1-9 receive two energy units, VIP levels 10-18 receive three energy units and VIP levels 19-20 receive four energy units (thus the Trainer can have up to a maximum of thirteen energy units).
The total amount of energy a Trainer can have is unlimited. Any energy that is gifted or rewarded is added to the current amount, which displays in green if overloaded (more than the base amount). Overloading stops the automatic regeneration of energy until the amount drops below the base amount again.
It costs 1 unit of energy to engage in a battle on Normal Mode and two units of energy for a battle on Heroic or Nightmare mode. In addition to its use to start battles, Trainers can also use Portal Energy to hire a friend's Dragon to aid in battle.
The Portal Energy cost needed to hire Dragons from friends before battle depends on the level of the hired Dragon. To engage in battle with a hired Dragon, the energy amount specified next to that Dragon must be had, plus the amount of energy required to commence the battle.
See Battle Energy for more information.