mposki,
You do not give us enough information to be able to give you a reasonable guess. Was your charging the only electrical activity occurring at your home at the time? What sort of charging configuration do you have at your home? If you have an HPWC and dual chargers you can get 80 amps with 240 volts, or 19.2 kWh. Two hours at that rate is pretty darn close to 40kWh.
If you do not have dual chargers or only used a 40A source, then your usage for two hours would be about your 20kWh figure.
Was this midnight to 2AM or noon to 2PM?
I would also set my charging display to show kilowatts added, and not mileage. Then after charging is completed, compare the amount recognized by the car to your PG&E usage graph. Make sure you are not using any other household appliances, and turn off your HVAC unit during your test period. That might give you better insight too.
We live in the Central Valley, and we used to leave the HVAC on all night, and sometimes it would kick in for 30-40 minutes while we slept.