When you attack Invaders and Uber Invaders, your Hero's energy will decrease by a certain number of units, and will subsequently be restored to its base level.

For example, the base level of your Hero's energy is 1,000 units. By using the "Hero's Energy 2,000" item, you increase the current amount of energy tо 3,000 units. Let's say, after that, you attack an Invader and spend 2,500 units. This will result in a decrease in the amount of energy down to 500 units. In time, your Hero's energy will restore, but only to the base level of 1,000 units.

You can restore your Hero's energy by:

• Waiting for a certain amount of time, until it is restored to its base level automatically
• Using the Hero's Energy item
• Leveling up your Hero, which restores energy to the base level automatically

The maximum possible amount of Hero's energy is currently 5,000,000 units.

You can find more details about the Hero and Hero's energy via this link.