And, of course, you can always replenish your energy for crystals.

Energy is the pep you need to develop your Island, clear ground and help tourists. You may also need energy to participate in certain events.
The energy bar holds 25 points of energy. Every three minutes, you get one point of energy back. When you reach a new level, your energy is fully restored.
And, of course, you can always replenish your energy for crystals. If you don't have enough energy for an action, you will be offered to restore it for crystals immediately. Note: in such cases energy is restored, but without the units you needed for the action. Those units are taken at the same moment.
To increase the Energy scale, you can purchase a special Mint Tonic offer in the game. After that, the maximum amount of energy that you can accumulate will always be equal to 28.