A fusion reactor.

The Fusion Reactor is the most advanced of all sources of energy. It is very similar to the Solar Plant, but produces more energy. It is like the Solar Plant; it produces more energy every upgrade, although, the more upgrades, it will produce a higher amount of energy than the Solar Plant. It is highly recommended to upgrade this the more upgrades, buildings, resources production there is. Just like the Solar Plant, the more upgrades on the Fusion Reactor, the more energy produced.