Modern ones work, like you said, down to -20F. One important caveat however is that they don't not-work below -20F, they just fall back to "emergency heat" aka

Modern ones work, like you said, down to -20F. One important caveat however is that they don't not-work below -20F, they just fall back to "emergency heat" aka resistive coils. While of course emergency-heat uses far more power that power can still be green. The biggest problem remaining is cost, $20k+ for a 24 SEER heat pump is just bonkers.