bobert -

I can believe that propane heat is more expensive in many, if not most, places. Back when lots of people at the office were discussing utility costs, in our area propane was a little cheaper; but the last time I filled an RV bottle the price had doubled since the previous fill. Natural gas prices, and electrical rates actually do vary quite a lot across the country, and not always in step with each other, so the calculations have to be done based on what's available locally and on how long you guess it will take the RBs to lobby another rate hike through.

Regarding how cold the outside can get and still get heat on the inside, the heat pump has to be able to get the cold side heat exchanger colder than the outside air, in order to take heat from it; and it has to be able to make the hot side warmer than the air around it to dump any heat into it. Most heat pumps have "cutoffs" - usually fairly simple thermostats - that limit the temperature range they'll work in, but most of them have a "thermal capacity" well outside the cutoff range. If you're concerned about long life and/or the possible negative effects of operating outside the normal (specified in your user manual?) range, a heat pump that works "too well" might be ready for some maintenance.

For the heating cycle, the cutoff used on the cold side is rather variable, depending on manufacturer, but for the cooling (air conditioning) cycle it's common to shut off when the evaporator gets to about 40F (~5C) to prevent ice formation that makes the thing work extra hard due to near zero efficiency (and to avoid wet carpets that people sue ya for when the ice melts).

Burying the outside heat exchanger below the frost line can significantly extend the difference between indoor and outdoor air temperatures that can be achieved, since in most areas the heat exchanger is then working in/out of a fairly constant temperature that's stable and "more benign" than the actual air temp. As my friend did many years ago "submerging" the outside exchanger in deep enough water works even better. The frequency of use of these additional design methods varies a lot with where you're located, and outside of areas where experience with buried/submerged exchangers has been accumulated contractors may not have the knowledge or equipment needed to use them.

John