How long to recharge the batteries
If your electric circuit is 15 amperes and the voltage is 110V, then the GM charger should be around 12 amperes (to avoid the breaker to open).
So, 110V * 12amps = 1320 W-h or 1.32 kW-h
Then, to recharge 8 kW-h, the time needed will be 8000/1320 = 6.06 hours
Claude