Not sure what's meant by "6kwh" chargers. Charging rate is measured in kW, a unit of power. Energy is measured in kWh.
Charge at 6 kW * 1 hour --> 6 kWh came out of the "wall". Multiply units and values. 6 kW * 2 hours --> 12 kWh
Not sure what's meant by "I charge as much as I can (3/6kwh) while in town, but if I'm looking for a full charge, I will leave the car charging in the public garage and take the bus home for a few hours and return when it's charged."
If you only got 3 or 6 kWh, that's not much energy in relation to whatever you car's battery pack usable capacity is.... 75 kWh?
What are "TYPE 4" chargers?