I haven't seen it hit -20F for years - and I'm in northern NH (just north of Mt. Washington).If you want to have some heat for those extreme cold days, -20°F to -30°F or more, there is a product called Sweeter Heater. . .
In the early '80s when I was home (here) from college one Christmas it hit near -40 overnight and topped out at -20 (I remember it because that was so unusual, maybe the coldest day in my life here!). In the 1990s when I lived in a valley in Danbury CT and the cold air sank in on clear still nights I'd call my parents in the morning and we would compare temperatures - several nights were -20F both places.
But I've lived back here for 4+ years now and I can't remember seeing it nearly that low once in that time. As far as I can remember over the last several years it rarely goes below -10F now.