Unregistered solar is huge in the developing world. In the Philippines, basically no one tells the utility they installed solar, because the net metering application is so expensive, comes with tons of red tape (read as opportunities for government officials to extract bribes), and only gives you billing credits anyway. So people just install batteries and automatic transfer switches. Some even disconnect from the grid completely, or never connect in the first place because it was too expensive to get the line installed. Automatic transfer to a generator is now very simple, and with LPG generators, extremely clean and low maintenance.

As I understand it, this is very common across much of the developing world now. So the tracking of how much solar energy there is in the world is almost certainly a lower bound on the reality.