@Pieter
if you do not have connected anything on AC_in except the grid, though all PV inverters and all loads on AZ_out, than the internal power meter is enough and you do not need a separate grid meter.
Pls mind an VM-3P-75CT or an Em540 is much more precise. But you can add them later.
If your MP2 is net parallel , though loads/PV inverters on AC_in, than V you need a grid meter.