You have a great opportunity to get additional benefits from the spent energy − a piggy bank. How does it work? When you spend energy collecting resources, the piggy bank fills up.
You can check how full the piggy bank is by clicking on the icon at the top of the screen − the scale will show how close you are to the goal. As soon as the scale is full, you can purchase a special offer.
After the offer time has expired, the promotional offer will disappear and the piggy bank can be refilled.
📌Note: the piggy bank is filled not with your actual energy, but through the actions performed in the game.