I have the following energy weapon on a ship I am designing:
Washington Armaments 15 cm C3 Plasma Carronade (1) Range 60,000km TS: 4,491 km/s Power 6-3 RM 10,000 km ROF 10
My understanding is that this weapon requires 6 energy to fire and charges at a rate of 3 energy per 5 second increment. So, a reactor which outputs 3 power would be ideal for powering this weapon, assuming no other energy weapons are present on the ship in question.