If you need more Campaign Energy, you can purchase it using Diamonds.

To buy Campaign Energy:

1. Tap the Green Can icon in the top-right corner of the screen.

2. Select the amount of Campaign Energy you'd like to purchase.

3. Confirm the purchase using Diamonds.

Campaign Energy also regenerates automatically over time at a rate of 1 Energy every 4 minutes, as long as you are below the maximum Energy cap.