newbieuser said:

It might be bit silly but how does a main board/busbar etc know when to draw power from the grid or an inverter?

It doesn't, as Simon Barker says it's all done by the laws of physics.

The PV inverter, along with Ohm's Law and what we know as "Kirchhoff's voltage and current laws" decide that for us.

Basically, the inverter provides a slightly higher voltage than the grid, which means (using the above "laws") that if local loads are using less power than the grid, current is delivered to the grid, but if local loads are using less power, then there is a net "import" of power.

To prevent unnecessary raising of the grid voltage, the inverter tracks the voltage it's connected to, and lowers or raises its own output voltage accordingly.

newbieuser said:

It might be bit silly but how does a main board/busbar etc know when to draw power from the grid or an inverter?

It doesn't, as Simon Barker says it's all done by the laws of physics.

The PV inverter, along with Ohm's Law and what we know as "Kirchhoff's voltage and current laws" decide that for us.

Basically, the inverter provides a slightly higher voltage than the grid, which means (using the above "laws") that if local loads are using less power than the grid, current is delivered to the grid, but if local loads are using less power, then there is a net "import" of power.

To prevent unnecessary raising of the grid voltage, the inverter tracks the voltage it's connected to, and lowers or raises its own output voltage accordingly.