How much battery power does this use, and this time of year with lights and heating what kind of milage are you getting from a full charge?
A. Very little, barely measurable.
B. Thats an odd one to answer accurately. Get in the car on a cold morning and the range maybe showing as 20% less than it would on a mild day, but after driving for a bit the battery warms up and 'conditions' and the range is pretty much restored. Provided the car isn't being used for daft short journeys where the battery has little time to condition the actual range, as opposed to the displayed range, is little different to what it might be in June.
In the van I could run the heater all night set to max and use about 2 litres of diesel.