31.250cm C5 Soft X-ray Laser (1) Range 384,000km TS: 6,667 km/s Power 26-5 RM 60,000 km ROF 30
This laser needs 26 power to fire. Your capacitors feed it 5 power per 5 seconds so it takes 30 seconds to fire, a 4.5 capacitor would also take 30 seconds to charge it and your reactor could generate .5 power less per turn
Twin 15.0cm C5 Soft X-ray Laser Turret (2x2) Range 360,000km TS: 20000 km/s Power 12-10 RM 60,000 km ROF 10
Each of the lasers in this turret needs 6 power to charge, each of them has a capacitor feeding 5 power per 5 second turn, you would get the same rate of fire with a rating 3 capacitor meaning you could have 3 less power generated from your reactor for each laser if you gave them a capacitor of 3 and still have the same rate of fire.
Essentially if your capacitors use more power than the laser needs that power is wasted. So for the spinal laser it is nor important but you are using 167% of the power needed for each 15 cm laser. While reactors are not that big using an oversized capacitor also increases the cost of the laser , so ideally you should have a capacitor rating either equal to the power demand of the laser for a ROF of 5 , or half for rof 10 or a third for rof 15 etc.
