From Starfleet Commander
Energy is used mainly to power mines. If the mines lack enough energy, they will produce at a proportionally reduced capacity until you produce more.
- You have a level 3 Ore Mine which requires 40 energy to operate at full capacity, which would produce 119 ore per hour. However, you have only a level 1 Solar Plant, which produces 22 energy. Therefore your ore mine will produce (22/40) * 119 = 65 ore per hour. As soon as you upgrade your Solar Plant to level 2, which produces 48 energy, your ore mine will operate at full capacity again.