Power lines carry electricity over distances. Power jumps across streets and from one building to the next, but for longer distances you need to build power lines. Start and end a power line at the perimeter around a building.
The power line, despite being an electrical structure, does not actually produce electricity. Rather, power lines are meant to distribute power of one general area with a functional power system to another unconnected area.
Power lines are not necessary to transfer electricity from building to building. All buildings within a small range distribute electricity to each other. Power lines are used mainly to provide electricity to areas that cannot be connected by the power grid from the small range.