When making an efficient reactor group, the basic rules are that:
1. For the reactors to be considered connected, they must share a face; those that only touch diagonally are not connected.
2. The most efficient groups are those in which every reactor contributes to make the imaginary "box" that contains the whole group bigger. This means some of the most efficient shapes are simple straight lines, as well as T, L, and X shapes.
It's important to keep this "box" concept in mind. If you have, say, ten reactors, you could make a 10-block line, or a 6x5 L shape, or a 4x4x4 corner shape. What's important is that the "box" is as big as possible, because it is included in the power formula. In these three examples, the sum of the box's sides is 12: 10x1x1, or 6x5x1, or 4x4x4. But if I used those 10 reactors to make a C shape that was 4x4, then the "box" containing it would only be 4x4x1 = 9 and the power would be lower.
It's kinda hard to explain with words, though. Suffice it to say that, if you don't need a lot of power and you have space such as on a planet, a simple straight line of reactors will be perfectly efficient.
Also, don't forget that if you're connecting factory enhancers to your factories, those increase the power they consume as well.
I am re-inhabiting an abandon base my buddies left due to pirate attacks. I haven't attack at all but I am waiting and in the mean time making the place look nice. I am going to build a zen garden to meditate and let the ideas come to me. I feel too crowed at their space station.