A zero sized sink hole in space-time that can be filled with energy. In theory, and as long as the quantum field is strong enough, any amount of power can be contained within one. When in use they generate EM pulses as a side effect.
They are described as a black cylinder, one metre in diameter and eighty centimetres high. They are seen as the ultimate storage devices, and very few people used them as they didn't need the amount of power they produced. CST uses them as a backup power source for their wormhole generators. They are also used as power sources for starships.