Activating the arc pylon on the workshop's roof | Mission 8 - The Loyalists Dishonored Guide
Last update: 11 May 2016
Since you already have the blueprint that you have collected from room 202 of the pub's building, you can talk to Piero right away and give them to him.
Now, you need to make an important decision, because you need to decide on how you want the pylon to work. Selecting the upper-right dialogue option leads to enemies being electrocuted right after the pylon activates. The dialogue option in the middle will result in the pylon's stunning opponents, which is far better an option.
New Optional Quest: Bring a whale oil tank to the Arc Pylon on the roof of Piero's workshop
This particular puzzle is an easy one, and it consists of several steps. First of all, return to the workshop's first floor and interact with the device to the left (the upper screenshot), in order to "summon" an empty tank.
Lift the tank now and carry it over to the device to the right (the above screenshot). Pull the lever to fill the tank with whale oil.
Now, carry the filled tank over to the external balcony. This is the most troublesome part of the puzzle, because you need to carry the tank over to the roof. Try and perform longer leaps, while minding tallboys.
Once you reach the roof, put the tank down for a moment, and pull on another lever (the above screenshot), which will result in all three slots being exposed. The only thing remaining here for you to do, is put the tank you've carried over here, into the empty slot.
Wait for the arc pylon to activate, and either kill or stun (depending on your earlier decision) the enemies around. Now you can barter with Piero in order to replenish your resources and acquire the upgrades that you ignored earlier.