how does a main board/busbar etc know when to draw power from the grid or an inverter?

You could equally ask, in a simple conventional setup, now does the DB know how much power to send down to the individual outgoing circuits? Clearly the outgoing protective devices can't regulate the power flowing through them (they just switch off if something goes wrong) and the power must vary as individual loads are switched on/off (or dimmed). Or how a plumbing system knows how to send the right amount of water down a pipe when a tap is turned on. The answer's the same - Ohm's & Kirchhoff's Laws (for electrics) (basically the connection of a load causes a small voltage (or pressure) drop which allows a current to flow until it all reaches a dynamic equlibrium, change things and a new equlibrium if found.

Now just think of PV inverter in the same way, other than the currnt is flowing in the other direction - i.e. it's a negative load - so exacty the same principle, but with -ve numbers (i.e. you get a small voltage increase from the DB to the inverter).

   - Andy.

 how does a main board/busbar etc know when to draw power from the grid or an inverter?

You could equally ask, in a simple conventional setup, now does the DB know how much power to send down to the individual outgoing circuits? Clearly the outgoing protective devices can't regulate the power flowing through them (they just switch off if something goes wrong) and the power must vary as individual loads are switched on/off (or dimmed). Or how a plumbing system knows how to send the right amount of water down a pipe when a tap is turned on. The answer's the same - Ohm's & Kirchhoff's Laws (for electrics) (basically the connection of a load causes a small voltage (or pressure) drop which allows a current to flow until it all reaches a dynamic equlibrium, change things and a new equlibrium if found.

Now just think of PV inverter in the same way, other than the currnt is flowing in the other direction - i.e. it's a negative load - so exacty the same principle, but with -ve numbers (i.e. you get a small voltage increase from the DB to the inverter).

   - Andy.