Basically the Coefficient of performance (COP or SCOP for seasonal version) just keeps going down with the temperature (under 1 means resistance heating is bett

Basically the Coefficient of performance (COP or SCOP for seasonal version) just keeps going down with the temperature (under 1 means resistance heating is better) https://en.wikipedia.org/wiki/Coefficient_of_performance https://en.wikipedia.org/wiki/Coefficient_of_performance Models made for cold climate work to around -30c. Wikipedia has okish writeup about their functionality in cold climates https://en.wikipedia.org/wiki/Air_source_heat_pump#In_cold_climates https://en.wikipedia.org/wiki/Air_source_heat_pump#In_cold_c... Don't really have any good sources in English for this (most reading I have done on this is in Finnish) For example here are some excerpts from a report done by VTT (the Finnish governments research center) https://www.scanoffice.fi/vttn-testiraportit-ilmalampopumppuvertailu/ https://www.scanoffice.fi/vttn-testiraportit-ilmalampopumppu... If you scroll down you can find some pics "lämpökerroin" is COP and "ulkoilman lämpötila" is outside temperature. From that you can see that the combo of Mitsubishi Electric MSZ-LN25VGW + MUZ-LN25VGHZ still holds a COP of 2 at -30c. On the models not made for such climate/cheap ones you can see it drop to 1 (or below)