In first approximation, the "demand" you see in a blackout is the maximum your factory could conceivably consume, if every machine was running, every inserter swinging, every roboport charging 4 bots, and so on. That has nothing to do with your third nuclear plant, it is an artifact of be...

I built 2 nuclear power supply plants, each consisting of 6 power reactors and their required supporting structure. I did each one at a time. I had no issues. Sometime later I built a 3rd plant, using the same blue print I used for the first two. With the structure partially completed I noticed I ha...