In my early days in starmade I always tried to get the most efficient reactor I could get with less power blocks. I tried every way to do this. L shapes, crosses, even some weird donut like shapes. Then I started fighting, and all of that changed.
The problem here is you need to consider what happen to your ship while fighting. A big chunk of your ship just got obliterated, and now you\'re powerless? That\'s because you power reactor just got divided, and the benefits from its design are now your main problem. As an example, a design like fstratzero showed some post above, with 10x10x10 dimension, can produce 6500ERS (Energy Regen per Second), but just by destroying the connecting block (the origin of the \"cross\") you\'ll end with only 3840ERS, and that could mean no more power for PewPew!
Your main concern while designing your power should be, not the most efficient reactor, but a reactor that is versatile enough to sustain some power while still managing to power your ship after some damaged.