From Halopedia, the Halo wiki
The Generator is a building in Halo Wars 2. It serves as the replacement for the UNSC Reactor. The Generator provides power to upgrade units and tiers, unlike Reactors, which unlock prerequisites to research new tech. Unlike the Banished's Extractor, the Generator creates energy in more more natural ways, such as wind and solar power, instead of harvesting power directly from the Ark.
Each firebase has an integral power plant to meet immediate needs, but additional Generators are needed for the accumulation of Power used for unit assembly and purchase of upgrades. The Advanced Generator upgrade significantly improves Power output. The older "pod" bases deployed by the Spirit of Fire used hydrogen-burning reactors for power, which required expensive and bulky fuel storage and safety systems. However, new firebase structures require far less power to function compared to the legacy designs, allowing Isabel to adapt modern UNSC vertical axis turbines as a more sustainable replacement.
On gameplay, the first Generator will always cost 200 supplies. However, every time the player requests an additional Generator, the player must add 300 more supplies for each Generator player buy (200, 500, 800, 1100, etc.) therefore, making a high amount of supply flow important.
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