My understanding is the PW2 gateway (inverter) monitors all 3 phases but the PW2 battery is only single phase. The gateway totals the amount of electricity being imported and exports the same amount to the grid, effectively zeroing out the import. e.g. If I am using 6kWh across 3 phases, and my solar is producing 3 kWh, I am importing 3kWh from the grid. (1 kWh per phase) However, the gateway will simultaneously export 3kWh on one phase, giving a net import of 0kWh from the grid. I found this diagram that might make more sense. Currently I have a 3 phase solar inverter and have the option of adding either a single phase or three phase inverter if I add more solar. The only real difference is if I want blackout protection, I need to add a single phase inverter. However, from a load balancing perspective, another 3 phase inverter makes sense. BTW, the PW2 and gateway are AC coupled, not DC.
