Energy refills with time and fully restores when you level up. Putting fires out gives you 2 energy points each time, helping neighbours in their cities – 1 energy point. Some buildings produce energy.
If you totally run out of energy, you can watch a video to get 5 energy points, ask your neigbours for it or buy energy with superbucks.
You can also win energy packages for playing SuperBINGO quests and spinning the Wheel of Fortune; when you call your sleeping neighbours back and they return you receive energy supplies as well.