There is a limit to how much rooftop solar PV can be used in a local area, i.e. the area served by the local area transformer. This is because the local area transformer only works one way. When the amount generated by local rooftop solar PV approaches the amount being consumed locally there cannot be any more generated, because it's not possible to send power backwards through the local area transformer to the grid. I believe the power company can even send signals to inverters telling them to limit their output if this situation is likely. The obvious solution is to make transformers which can feed back to the grid.