Energy is needed to play battles (levels, scenarios, maps, whichever you call them) and claim level taxes. For each of these actions you need 3 energy. If you want to always get more money, you need to accumulate energy to get them at once.
By default you have a maximum of 120 stored energy, and recover 1 energy each 6 minutes. Thus, your energy storage is big enough for 12 hours of inactivity. Energy bar can be exceeded with Food items, displaying green.
These limits can be modified temporarily (for a few days) with the Fairy Alliance feature, which costs diamonds. The Fairy Moxie will increase the maximum energy to 200, while the Fairy Connect will increase 10% the energy recovery speed (which should bring 1 energy each 5 minutes and a half). See full costs on the Fairy Alliance page.
There are four items that serve to increase your energy (if you are not at the limit):
- Ham: Also called Turkey Leg. On use it will grant 1 energy
- Burger: On use it will grant 5 energy.
- Apple Pie: On use it will grant 20 energy
- Chicken Wing: Also called Turkey. On use it will grant 30 energy
The only one that can be dependably and freely harvested is the Ham. You get around 10 of them each day through your Online Reward and Daily Tasks. You can also get Ham at random from the Daily Login Reward of the Reward Center.
All of them can be gained from the Tavern at random, but except for the Ham, their probability is very low.
You also get a few Burgers, 3x Apple Pies and 2x Chicken Wings from the Daily Grind, but this is a once-in-the-game event.
The main way to access the energy items is to buy them at the Marketplace, usually for diamonds, though they sometimes appear on the Black Market. Even with this, they are limited in number. You can only buy 4 "Turkey" (Really Chicken Wing, 30 diamonds) and 6 Apple Pies (25 diamonds) each day. VIP 5 doubles this amount. So for the exorbitant price of $27/day, you can spend a total energy of 240 (free) + 10 (Ham) + 120 (Chicken/Turkey, bought) + 150 (Apple Pies, bought) = 520 energy/day.