I'm in Northwest Montana. My ground source heat pump doesn't struggle until the highs outside are -20F (actual, not wind-chill). I have the backup heat strip, but the breaker is off. I don't know when it would turn on, I just wanted to know it wouldn't without me knowing it.
I'm in Canada at a similar latitude with ground source, resistance heating normally kicks in at about -25C (-13F) or so, just a few hours on the coldest nights, doesn't cost much. I could probably leave the breaker off too, I wouldn't mind it a degree or two colder.
Ground source is often the only choice here. While air source can technically work well down to these temperatures, much of the available equipment doesn't suit some homes.
Ground source is largely going to maintain capacity independent of the outside temps, so the resistive would turn on when the heat pump isn't keeping up with the heat loss.
