It should be at least 15 degrees warmer inside (with a front door on) than outside without any extra help. Our winters are quite variable. Two years ago we got heavy snow just after Thanksgiving and it stayed until March. Last year not so bad, and this year they are saying El Nino will give us a milder winter. I hope so!
During a bad winter we can end up in the teens (F) for long spells. But it's the high winds that are such a problem. we are sheltered on the south and east by trees, but the view to the west is a clear shot to Colorado. And boy, do the winds come through.
I'm also toying with the idea of putting a water butt inside to water with through the winter. With a stock tank heater, the water will always be about 40 degrees, warming the inside during the nights.
