http://4.bp.blogspot.com/-tdK_AMaZ9pg/Vb51WrvPDhI/AAAAAAAAD5...
Which is quite a lot of the country.
Light blue is a typical Winter low point in the -10°F to 0°F, which means you will see -15°F or lower often enough to worry about it.
https://www.energystar.gov/ia/partners/bldrs_lenders_raters/...
Though heat pumps are unique in that they produce less heat as the colder it gets - A few hours of -15º every few years shouldn't be the primary consideration in spec'ing a system that still produces 75% of its heat in that worse case.
I live in that blue area and ran through all of the math and considerations recently - I pulled the hourly temperature data for 6 years. Of the 94,000 data points in that period, a total of 26 hours were below -5º: https://imgur.com/a/P7A3kan
> Our modeling finds that even if Focus incentivizes 800,000 heat pumps with electric resistance backup (10 times the number of heat pumps as it did furnaces in the past four years), the state will still be able to meet its electricity demand with currently operating power plants, even on the coldest days. Depending on the efficiency of the heat pump, in-state winter generation capacity would still exceed peak demand by 1,400–4,300 MW on the coldest day.
https://rmi.org/three-questions-wisconsinites-are-asking-abo...