Every so often a Piggy Bank may appear on the right-hand side of your screen. As you use Energy to clear resources, an equal amount of Energy is deposited into the Piggy Bank.

The time limit for this is 7 days, then the Piggy Bank will disappear. At any point during this time, you can purchase the Piggy Bank for the Energy inside, but the best discount is when the Piggy Bank is full! Once it's full, the offer is valid for only 3 hours, so make sure you buy it before it expires!