RonDawg said:
I used "below freezing" to denote some point in the thermometer where it becomes inefficient. As the information I got varied quite a bit on the actual temperature, the last thing I wanted to do was quote an exact temp. But most of the sources that I saw quoted temps somewhere below 32 F, thus my use of the generic "below freezing."
You are failing to recognize that heat pump technology has changed a lot in the last 10 to 15 years.
The traditional home heat pump did end up depending on resistance heat somewhere between 17 F and 32 F.
But Japanese and European manufacturers began making very efficient mini-split heat pump systems about 20 years ago. It took a while for them to go into common use in the US, but within the past 5 to 7 years they have become the accepted correct way to do heat pump home systems in the US.
They operate very efficiently to much lower temperatures than the old style home heat pump systems. Some of them do not depend on resistance heat until temperatures as low as -5 F to 5 F. Based on the chart on the LEAF hybrid heating system, it apparently only depends on resistance heat below 5 F.