COP is the ratio of moved heat to electricity used. Modern air-to-air heat pumps can maintain a COP of 2.5 or more below 5 def F. The 2.5 COP is important because it’s the efficiency cutoff where, above that, even using methane (natural gas) to generate electricity, send that electricity to the home, and use that electricity to power a heat pump, you will use less methane than you would if you burned it directly for heat in your home.

A COP greater than 1 will still be better than resistive heat, and many modern heat pumps can maintain a COP above 1 well below 0 deg F.

Now, wherever you live, methane could be subsidized for homeowners, so the direct costs to the homeowner don’t pencil out directly at the 2.5 COP cutoff, but that’s no longer a question about efficiency, but around local government not correctly pricing methane.

EDIT: there was a week in January where it stayed below -10 deg F where I live, and my heat pump kept my apartment at 70 deg F with no issues, and only needed to run 27% of the time.