That's one of the worst manuals I've ever seen. I guess one of the voltages is that of the battery. Calculating the battery capacity might be possible but that depends largely on the battery type and I don't know how you get that calculated.
I would try to send some of these bytes to the inverter and see what you get in return. If it matches anything in that "manual" you might try the next parameter. Good luck. If the firmware is as good as the manual you'll fail.