We went with a guide through several favelas, including this (the largest) one - Rocinha.  You can see how simple the structures are and how they are just stacked on top of each other.  Need power?  Just splice into any available line as you see many have already done.  Organized crime protects itself in these communities, but violence is a regular event - you can see the black flag hanging from the wire in the middle of the photo.  It represents a member killed here recently.