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.
2011 535i x-drive GT, 2014 i3 BEV
Soon (hopefully!) A 2021 X5 45e will replace the above