There is DC high cutoff (battery fully charged) and DC Low regain (Buffer backup voltage) points in solar PCU.
Once Solar PCU is with PWM/MPPT Charge controller (MPPT CC), will Charge batteries simultaneously along with Grid Charger.
At this time load will be on grid. Now PWM/MPPT CC will charge batteries through Solar power.
Load will be on grid. When battery voltage reaches DC high cutoff voltage (Battery is fully charged).
Then Solar PCU disconnects grid supply and Load is transferred to Inverter. Now Solar gives power to inverter and for battery charging.
Solar PCU will work through solar until solar is healthy. When solar power not sufficient PCU will take deficit power from battery.
When battery voltage reaches DC Low regain (Buffer backup voltage) Solar PCU will shift load from Inverter to grid. Buffer backup is for emergency use in case of grid power failure. Once Solar power is available then PWM/MPPT CC will charge the Batteries and the process continues.