A power plant is required for all spacecraft. The power plant on a spacecraft consumes hydrogen to keep the spacecraft's electric capacitors charged. The amount power plant volume determine how quickly the capacitors recharge.
A basic spacecraft, one with only a helm station, fuel cell, life support, and gravity drive can run off a tiny power plant, even if the spacecraft has over 200m³ gravity drive volume at full throttle. A more than tiny power plant is required if the spacecraft need to operate FTL drives, sensors, shields, energy weapon turrets, or weapon bays.
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|Update 2017-01-17: Design Analysis