Fuel

Fuel is one of the currencies of the game, and is used to do battles in the War Zone.

This will be your main limiting factor to earn levels.

Use
Every click on the central zone (the Battle) costs 0.05% Fuel and does your DPC (Damage Per Click) to the enemy.

If you use either the consumable "30s 2x DPC", or the premium upgrades Double (2x) or Triple (3x) DPC, then every click costs 0.10% (for double DPC) or 0.15% (for triple DPC). See Battle for more details.

Refuel

 * Fuel is topped to 100% every daily reset
 * You can get extra fuel as a reward from daily tasks
 * You can get a full refuel from Spin the wheel

Time Challenges
The main source of extra fuel are Time Challenges. You get 5 from the daily reset. You can get 41%-63% extra fuel from each one of these.