The engine coolant has to stay at the right temperature regardless of whether you have the heater on or not. The colder it gets the more often it will run. The cold affects the battery power also. The electric mode maxes out at 30 or so miles max under ideal conditions. The power to keep the battery from running down has to come from either the gas engine or regen braking. Your plug in has a bigger battery than the regular hybrid but the electric motor has twice the power. Once you go into hybrid mode, you aren't going to see much benefit from the plug in feature. That's why the mileage numbers for the plug in are actually less than the regular hybrid.