PSA for newbies: many prefabs have wiring built-in
This is a PSA for new folks. I see a lot of camps from players just starting out, and they're using things like the Wildwood Tavern, Pittsburgh Palace, or Iron Mountain Anvil. The structures have wiring strewn all over the exterior and interior.
Many of these prefab buildings have a power connector on the outside. When a generator is hooked up to this electrical connection, the building has ambient power, and no further wiring is needed for lights. Most often these connections are at the "rear" of the building, opposite the main entrance.
Note that if you want to install things that need direct power (such as a vendor, weather machine, or fusion core charger) you'll need to string wiring to where that device is located.
I own a few prefabs, and the only one I have WITHOUT an exterior power connection is the Mega Mansion.