Today was the first time I did a longer road trip at below-freezing temperatures. Some interesting results.
The trip was from Ottawa to Kingston and back to Ottawa again, a total distance of 296.1 km. Except for the occasional small town, all the driving was between 80 and 90 kph. I avoided the faster highways to preserve range because of the lack of charging facilities at my destination. (I actually did pick up a few hours of 110V charging, which was nice because it padded my range margin.)
The total trip time was 4:05 hours, and the energy consumed was 49.31 kWh, for an average of 167 Wh/km.
Since the trip was going to be a good fraction of the car's nominal range, and conditions were not ideal, I took it easy on heating. I used the seat heater throughout because it's power draw is negligible (can't even see it on the power meter). When I needed to clear the windows I used the cabin heater very sparingly and only at its lowest power setting. I'd ballpark average heater usage at 3 Wh/km. (Continuous full power could have consumed 40 Wh/km, but that would have been too much heat anyway!).
The main nuisance of driving with the cabin heater off is that air still blows in via the vents. I closed the vents and put the air on the windshield to keep it clear, and turned on recirculation to reduce the flow rate (usually I avoid that in the winter but the Roadster didn't get humid - it was still sucking in quite a bit of outside air). With the seat heater set to low my back and seat were a little too warm, and with cool air bouncing off the windshield made my hands (no gloves) and face a little too cool. The net result was strange because I felt slightly too warm and slightly too cool at the same time; but overall it was reasonably comfortable and I could have stayed that way indefinitely without getting chilled.
Tesla's efficiency spreadsheet predicted 147 kWh at 90 kph. I've previously found that their estimates are quite accurate - in summer driving conditions, that is. Today's results imply that power consumption was about 14% above nominal. Accounting for heating we're probably looking at 11% in extra losses.
The temperature ranged from 0C to -5C. If we assume that Tesla's data is for 20C (room temperature) then the increased air density would account for most of the difference. The ratio is 1.316/1.204 (see Density - Wikipedia, the free encyclopedia) = 9.3%. Pretty close.
We've had a warm winter, and much of the precipitation has been rain instead of snow. Predictably enough a lot of roads around here are in pretty rough shape due to frost heaving. The secondary highways I was driving on were in really bad shape. I wouldn't be surprised if those thousands of bumps added a bit of extra power consumption.
I'm not sure about the rolling resistance of winter tires versus summer tires, but I don't think that was a big factor.
So the net result is 11% range loss, plus about 3% for heating. In comparison, Nissan Leaf owners say that they see a 1% loss in range for each 1C below 20C, not including any cabin heat. This would have predicted a 22% drop in range, and implies that the Roadster handles the cold quite a bit better.
One note: my battery pack was warm throughout. I have previously found that if your pack is cold-soaked well below freezing, you could lose up to 10 km extra during the warmup period.
Have other people seen similar results?