Technically speaking a solar PV system Installed on a domestic propoerty should never be larger than 3.68kW. Why? Simple physics. Your home’s ring circuit is rated to 16 Amps and electricity is supplied at 230 volts. Multiplying these two numbers gives you the maximum power that should be connected, 3,680W, or 3.68kW.
While I agree that efficiencies in the inverter will mean that the peak power of the solar panels is rarely reached, thats not the point. By installing more than 3.68kW you are risking the safety of your home on that rare sunny day when the panels over perform.
Your network operator has very clear guidlelines about the procedures for installing systems of more than 3.68kW – they’re called engineering recommendation G59. Trouble is that G59 is a long and costly application process that installers simply cant be doing correctly before they put your 4kW system up.
The consequences of not registering your system could unbalance the national grid forcing your network operator to take action. This action will be a request to disconnect your unregistered system, which of course means no more feed in tariffs.
Be careful when your installer tells you that you can fit a 4kW system on your roof.