[ianschmitz]

That’s misleading. Yes efficiency drops as the ambient temperature drops, but they are still quite efficient below freezing.

For example here’s one of the units I run at my home: https://ashp.neep.org/#!/product/56518. It has a COP (coefficient of performance) ranging from around 4 at higher ambient temperatures, down to 1.8 at 5f. So it’s just over half the electricity input per watt of heat output for me to run my heat pump at 5f.

There are newer units and units designed to run in cold environments that achiever much higher COP below freezing than my example above.