The losses are not in the wall box or granny charger, the losses are in the car. The wall box and granny are just glorified relays and safety interlocks, they are not chargers. There might be a few watts dissipated there at most.
The losses are in the vehicle where the charger lives. The vehicle’s charger takes the 230VAC converts it to DC then steps up the voltage with a high frequency inverter (that also provided the isolation between the grid and the vehicle) that is controlled in such a way to deliver regulated voltage to the approx 400V battery. It’s through those steps in the vehicle where the losses occur. On top of that the energy available from the battery is always less than the energy put into the battery because the chemical conversion process is exothermic and the heat generated during charging comes from the power applied.
These vehicle losses completely dominate compared to any minor differences in a wall box vs granny.
