Cold-Climate vs. Standard Heat Pumps: The 25°F Line
Why efficiency falls as it gets colder
A heat pump moves heat from outdoor air into the house. The colder the outdoor air, the harder that is, so both its heating capacity and its efficiency fall as the temperature drops. When a standard unit can no longer keep up, backup electric-resistance heat makes up the difference, and resistance heat is only 100% efficient, compared with the 200% to 400% a heat pump typically manages.
The 25°F line
Where a city's 99% winter design temperature is below 25°F, this site prices a variable-speed cold-climate heat pump, at a $2,500 premium over a standard unit, because such units keep much more of their capacity in deep cold and lean far less on backup heat. That applies to 2,444 of the 3,206 cities covered. The Northeast Energy Efficiency Partnerships (NEEP) keeps a public list of cold-climate models with their tested capacity and efficiency at 5°F, which is worth checking any quote against.
The extremes
Fairbanks, AK (-50.3°F), Bozeman, MT (-32.1°F) and Belgrade, MT (-32.1°F) have the coldest design temperatures in the dataset; even a cold-climate unit runs near the bottom of the model's efficiency range there. At the other end, Honolulu, HI (64.9°F) and Miami, FL (56.7°F) are so far above the line that a standard unit runs near the top of its range all year and a cold-climate premium would buy nothing.
Why backup heat is folded into one number
Rather than guess how many hours a standard unit spends on resistance backup, the model folds that into the single seasonal efficiency figure shown on every city page. An explicit backup-hours estimate would need hour-by-hour temperature data the model does not use, so it would look more precise than it is. The methodology guide has the full derivation.