Kalos Power Plant can be accessed from Kalos Route 13. There are 5 facilities on Route 13 but only one of them can be visited.
Apparently, if you haven't done the raid on the Power Plant, the game will still let NPCs past the workers stopping you from getting past them due to the blackout. However, if you play during nighttime, then the buildings will still light up at night even if you haven't restored the power to Northern Lumiose.
One of the facilities in the Kalos Power Plant in the game; this cannot be accessed.