Thanks, Jim, for your comments here. Heat pump performance does degrade as outdoor temperatures drop, although there are many cold climate models on the market that operate well down to 5℉, or colder. That said, as you noted, many heat pump installations do require some form of backup heat at low temperatures.

In my post linked to above, I encouraged readers to simply replace their air conditioners with identically sized heat pumps, and leave their existing heating systems in place for backup. Such systems will probably switch to backup well before ambient temperatures drop to those extremely cold temperatures. If their existing heating systems are based on electric resistance, then they would use that for backup heat, as you do. Other heating systems might well be based on a different fuel, such as natural gas, which in such cases would provide backup.

Again, thanks for weighing in here. Hopefully someone from an HVAC manufacturer is reading our exchange, and takes your advice regarding capacitors.