Here's some data from today at lunch. Captured (with scanmytesla) min battery temps, power consumed on front and rear motors and total power used (which also includes cabin heating). PTC heater, no heat pump. Outside temp was -14C. Temps in degrees C, energy in kW.
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I did not have the cabin temp notification turned on so I don't have the exact time. When I checked the cabin temp (I think around 12:12-12:14) it was already at 21C. It doesn't take long. The last line is from when I got in the car and put it in drive without moving. Active battery heating stops then.
20 minutes at an average of 11kW means a consumption of 3.7kWh, that's significant energy.
EDIT: Important note: subject to change with software updates... it was different last year.