I do not have anything definitive, but here are examples of what I have run into:
1. I had a base maybe 300 blocks away. You could not see the power plant from the base, used relays and lights about 14 blocks apart to reach. Power would stay on most of the time, but if I went far enough away for looting the power would be off on my return. And sometimes power would just turn off randomly while staying in the base.
2. Another base I had a 'horde base' about 250 blocks away, and had 30+ relays underground. I never had an issue with power although I only used it for horde night.
3. Yet another game, another horde base only 100 blocks away, the power plant was actually well within site. There seemed to be an invisible wall that if I crossed, the power would turn off 2 minutes after crossing it reliably. I performed quite a few tests to prove this. The power did not turn off immediately. Like a delay timer. I found the 'wall' through experimentation because what I had to do with that game was tear down the entire base and rebuild it inside that wall. I was not happy. There was a lot of steel to tear that down and then rebuild back up. On a happy note, I now discovered that the light saber is far superior for destruction than both the pickaxe and the auger.