Energy is required to battle against an opponent in PvP. You can have a maximum of 3 energies at a time. Regeneration of each energy takes 20 minutes.
You can also opt to avail energy using Gold. Here's how you do it:
There are two types of Energy Refills: 'x1' and 'x3'. Neither type will disappear until it is used. When you are out of energy, you need to tap on "Revenge" or "Find Target" to get a window "Get Energy". An 'x1 Refill' costs 8 gold, while an 'x3 Refill' costs 20. Energy Refills are the only Consumables not available for purchase from the Store.