it doesnt matter how big the other (only for visual) groups are unfortunatly
cost is calculated only based on number of groups. So it doesnt matter if you make 24 small and 1 big beam or 25 medium ones. actually, you'll have a better dmg spread with 25 medium one imo.
Edit: You could give each beam its own computer and trigger them all together via an inner ship remote p.e. That way, your power cost modifier is 1.0. This has a different downside though, cause you have to aim by moving the entire ship. so its only a solution for smaller / manouverable ships.