Empires & Puzzles has been balanced to use various energies for different activities. We want the players to experience the different aspects of the game. For example, if you're out of world energy, you can still raid other players. The energies replenish over time, generally ensuring that you have a full stock of energy every few hours. Raid and World energies can also be replenished with gems.
We're constantly looking into how to improve the game, including ways to get extra energy. Stay tuned!