It's usually about 35F to 22F daytime when I turn on the heater, so efficiency at low temperatures would be the main thing that matters. It doesn't get to -20F

It's usually about 35F to 22F daytime when I turn on the heater, so efficiency at low temperatures would be the main thing that matters. It doesn't get to -20F that often most years, but it does hover around -5F for days at a time on occasion. Current generation heat pumps are probably still worth it but ultra low temp performance would be nice.