The heater draws between 1 and 6 kW depending on how cold it is outside, how fast you're driving, and how hot you're trying to get the car. This results in a highly variable mi/kWh. For example, if it's 50 degrees outside, you're driving in city traffic, and you have the HVAC set to 65°, you'll use a lot less heater power than if it's 25 degrees outside, you're driving at highway speed, and you have the HVAC set to 75°. If you're a numbers geek, you can pretty easily use your driving speed and heater power consumption to estimate how much your range will be reduced (or mi/kWh impact).
My typical average efficiency is around 5.5 mi/kWh. In the summer without AC use, I get over 6. In the summer with AC use, I get around 5.5. In the winter without heat, I get around 5. In the winter with heat I get around 4.5. I usually only pre-condition if I'm plugged in, unless the temps are extreme. I am comfortable with the interior between 60 and 80 before I turn on the AC or heat, generally, but for short drives I'll tolerate more like 55-85. This is truly a case where your mileage may vary!!!
It's easy to minimize the impact of temperature on driving range by pre-conditioning when plugged in, dressing appropriate to the weather, and by adjusting your HVAC set temp to "less comfortable" to reduce the temperature differential from interior to exterior. Using the seat heaters when it's cold is MUCH more efficient than heating the whole cabin. If your hands are sensitive to cold, using driving gloves makes a huge difference in the cold as well. I have tinted my windows and leave them cracked when it's hot/dry, which helps to reduce the greenhouse effect while parked and also requires a lot less AC use when driving on a sunny day. If you want maximum range when it's hot, leave the AC off and the windows cracked about half an inch...you'll hardly effect the aerodynamics, but still get enough airflow to feel a noticeable improvement in comfort. If you want maximum range when it's cold, bundle up, use your seat heater, and keep a warm cup of something to drink on the drive.
Unfortunately, I haven't figured out how to use the heater without allowing the AC to turn on for defrost as necessary. That would be another way to improve range when using recirculating air in the cabin if it's not humid enough to fog up the windshield. It's not totally obvious when the AC pump is being used while you're driving, so this one is a bit tricky to understand so far.
Bryce
