There are two substations and a main station - Alfie wants you to go reset them. The problem is that the lights being out has created a strategic problem. Alfie has placed a trap in there - but he wants you to go in after dark to do this so that he can use the trap to kill a large number of the freakier infected.
Now BEFORE you make it night and take all that risk you should know that you can do the first stages - turning on the substations - during the DAY. So do that - turn on the first substation, then the second substation during the day.
Once you have both of them turned on, it's time to head to the Central Station to do the final re-connection.
When you arrive you will discover that (1) you have to find the way in, and (2) the place is a freaking zombie ZOO! There are a LOT of them inside.- so you might want to go in and thin out their ranks during the day then head into the shack with the master panel in it, close the door, and just wait for night to fall. That way you are right there where you need to be, no sneaking through the dark, no risk of having to deal with the undead, easy-peasy!
So yeah, once darkness falls you simply walk over to the panel and throw the switch - that will allow the trap to be triggered - at which point the shack you are in BECOMES a Safe Zone! So simply rest there until morning, then return to the Tower and have your chat to learn about a new quest called Searchlights - and you get a Blueprint.