Which one uses more energy?
There was an episode of Myth Busters that found in an ICE that the A/C with windows up received better milage.
I would think that with the MS's superior aerodynamics, A/C with windows up would have even more of an advantage than on mythbusters.
This was addressed in a review of the first long range tests of the Model S. It was an LA to Las Vegas trip without charging.
Motor Trend said in an interview that Tesla later told them that using the AC is more efficent than having the windows down.
I believe it was the video in this thread:
a little offtopic, but please don't refer to mythbusters... thats just a show and the "scientific" facts are in the most instances ridiculous
Similar topic: what do you think is the effect of venting the sun roof?
The windows vs. AC-discussion is also a question about speed. The AC draws the same amount of power regardless of speed (more or less). So if you're driving along at 5 mph or 100 mph, it amounts to roughly the same.
However, the windows are a different story. Driving at 5 mph, the drag resulting from having the windows down is negligable. At 100 mph, the resulting drag is significant.
At some speed X mph, the energy requirement of the AC will become less than the energy required to overcome the increased drag created by the windows.
The How Far Can I Go range calculator answers this question. It seems that at 65 mph, rolling down the windows costs about 10 miles of range and as might be expected this does not change with outside temp. The range cost of the AC depends on the outside temp and ranges between 8 miles at 70° outside temp to 15 miles at 90° outside temp. So it depends, but the "costs" are similar.
Windows down works best for cooling in the winter, and heating in the summer.
Funny I didn't try those conditions with the calculator, but I am sure you are right!