Ok, current state of play is: Because I have two inverters I need to read both Modbusses which explains why all the date I had was not credible. Also because each inverter has its own battery I need to pick up Battery 1 from Inverter 1 and Battery ONE from Inverter 2. (Doh!).
So far I now have all the data coming in with reasonable values with the exception of PV Output. Still searching for this. I have Total AC Output and Total DC Output but is seems to be referring only to the passage from Inverter/s to House and not from PV to Battery. Still waiting from Solaredge Technical support who seem to be almost further in the dark than I am at the moment.
