That is an excellent question. Unfortunately, I have been able to test this on multiple occasions. Tesla's are remarkably good for this type of scenario. I have been stuck in the canyon for over 3 hours and was able to keep the cabin warm while we all watched Netflix. In total I lost about 5 miles of range. This is considerably better than any ICE vehicle and you get the added benefit of not emitting harmful gases while idling. If you want to hear the full story I wrote about it here: https://medium.com/cotarie/stuck-in-a-tesla-for-over-3-hours-db675102be1e