Different take from everyone else in the comments. Wiring. There is something new being made popular called a balcony solar panel. It hangs off the side of your appt balcony and plugs in to an outlet. The electricity generated gets added to your appt's wiring. The problem is regulation. Most solar installs require agreements and inspections between you, the local city govt, and the utility company. I believe they're pretty popular in Germany because they set up the regulations to handle it. But many places don't want unregulated power generation. The concern being when there's a power outage, linemen need to fix stuff, and if they're expecting a line to be dead due to power being out, you can injure them by dumping electricity on the grid. My solar panels were inspected to verify when the grid goes down, they don't put any electricity onto the grid.