Battery conditioning, as started when you set a departure time, runs the 1Kw heater. But, if you also check the cabin preconditioning, I've seen the system drawing nearly 3Kw. Depending on the year and where you are, the OUC plugged into 120vac is at most a 12A device, so 12*120vac=1,440W. It should run the battery heater without affecting the battery charge. But, if you also want the cabin conditioned, you will not leave with a full battery.