My primary vehicle prior to receiving my Model S 85Kw was a BMW GS1150 motorcycle that I rode year-round. Needless to say, when I bought the MS I sold all my other toys but I kept a few choice pieces of riding gear. One is a jacket liner and gloves that I can plug into the 12V Accessory receptacle and have an even more efficient means that just the seat warmer to keep warm during the winter months. I did a little research and the power draw is minimal for these systems and I can attest that they work quite well. Based upon the posted specs the jacket draws approximately 6.4Amps at 83Watts which translates into 0.083 kWh, but I never have the jacket turned on high. I did not opt to purchase the Gerbing's jacket, instead I purchased the First-Gear jacket from CycleGear for much cheaper and the gloves are Gerbing's gloves bought on clearance at my local BMW bike shop.
As an aside, I will let my wife use the jacket and gloves on longer trips because she runs a lot cooler than I (been a long time since she moved to the US from Poland). The only time that we should require the gear is when the trips are longer than 200 miles with no charging infrastructure in the winter. We are planning a winter trip up to Angels Camp, Ca soon and there is as of today no chargers there, apart from an RV park something like 20 miles away.
The link for the jacket: http://www.cyclegear.com/CycleGear/Street/Jackets/Heated/brand/SEDICI-HO...