Some trucks, boats and airplanes use a separate gasoline or diesel heater such as the Espar for preheating the engine and for cabin heat. My guess is that about 8kw (~25k BTU) would be needed for the battery and cabin heat at 15F and 60 mph, that's roughly 135 w/mi extra which would burn about 1 liter or .25 gal/hr to heat the battery and the cabin while driving. With a timer it would eliminate the overnight range loss and have the car completely preheated in the morning while using very little battery power and fuel. I'm not sure what the additional cost would be, probably somewhat more than a second charger, but between overnight losses, battery heating, and cabin comfort, it could add well over 70 miles of range in cold weather. Other than a heater, I'm not sure what options Tesla has for cold climate driving. People aren't going to be happy driving for hours with the heat off at 55 mph, or stopping every 3 hours for over an hour and a half to recharge, even if there were enough Superchargers. Once the X becomes available and more people start wanting to drive to Grandma's with young kids in the car it's going to be even less acceptable.