Hi there. The increased power demand is from the staff working double time to make tons of Mirrors. At the moment, nothing is wrong with the reactor. If players decrease the power load through, nothing will happen. Though, you could say the machines will be less effective or even shut down, making the staff angry and try to get to the players.
However, if the players increase the power usage further, then the Reactor will go into a nuclear meltdown due to overloading, which can be a catastrophic way for the players to defeat the horror of the facility, if they can get out in time. This is indicitive by the overloading instructions found on Luzia's dead body. How long they have to escape is up to the Warden.
Let us know if that helps and how your playthrough of it goes!