Cool. But, how much of the US gets below 17F? I think it’s a lot.

17˚F isn't the point at which my heat pump stops working. It's just the lower temperature conventionally used as a point of comparison on the spec sheets for such devices. There's another column on the spec sheet for 47˚F (where my heat pump model is listed as having a CoP of 3.6).

And as for the question about colder climates: it's not a matter of whether an area can occasionally experience temperatures below eg. 17˚F. What matters is whether you'll ever experience daily highs that are cold enough to prevent heating the house.

The question is what the cop is at -20C though, which is a fairly common winter temperature, and you really need heat at -40

I was responding specifically to this bit: "Once it hits 0C the efficiency is very low, and by -10C is almost doing nothing at all." I've refuted that. If you want to discuss some more distant goalposts, I won't be able to offer firsthand evidence.

You are free to refute that.

Doesn’t make it wrong.

At -10C you are hardly pulling anything from the air, but you are spending electricity to keep your coils free of frost, as inefficiently as possible.

Do you not know how a heat pump works in cold weather?