A solar plant.

 The Solar Plant is the most basic source of energy, and is the most important building for starting colonies. It must be upgraded in order to construct and upgrade buildings once the colony is fully constructed. Resource production requires energy as well. If the Solar Plant does not provide enough energy for the upgrades, buildings, and resource, only the resources production is affected and automaticly set to a certain percentage of production that uses lesser energy than before. The more upgrades that are done on the Solar Plant, the more energy is produced. Although the Fusion Reactor produces more energy for the same levels. It costs a lot more to build.