Energy is the number of condoms you have.  Exactly, safe sex is important!

The current number of condoms is displayed in the menu on the left.  You have five condoms by default. You use one with every game session.  Energy is restored 20 minutes after the game is over.  If you’ve run out of condoms and need some more asap, you can always purchase more by clicking on plus icon or in the “Customize” tab.

Number of condoms

“Add more condoms” window

After choosing the number of condoms, you can buy them or add them to your Wish list.

“Purchase condoms” window