That is the case only if you want to give your surplus back to the grid.
If you avoid that, you are only limited to a maximum power of 20kW of solar panels installed.
I recently got a 10kWh system installed and it should cover about half my usage. I live in meh-irradiance Canada. Full coverage of my bill in summer, pathetic generation in the summer. So yeah, sunnier places like Italy 10kWh would be perfect -- especially for places with heat pumps and warm winters.