Modern heat pumps can be up to 500% efficient (using 1kWh of energy gives 5 kWh of heat) at optimal temperature.

Yes, I went with "300 or more" because that's a more typical number for a run-of-the-mill unit.

And even the low-temperature-capable heat pumps reduce toward 100% at the low temperature end, IIRC.

Yep, here's a spec sheet for a unit I'm looking at as a drop-in ducted replacement for my existing system -- it has all of the models listed, but I'm looking at the 4-ton unit (12k btu per ton, so 4-ton = 48k BTU, hence the MDU18048): https://mrcool.com/wp-content/dox_repo/mc-uni-perf-ss-en-01....

It takes 7kw at 17ºF to provide 48k BTU. There are 0.293W/hr per BTU so 7,000W = 25k BTU "in" and 48k BTU "out" or a COP of 1.9. At -15ºF, it's using 5.9KW but can only produce 28,500. So a COP of ~1.4. Still better than electric resistive heating but not by much. At 40ºF, the COP is more like 3.3 which is in line with the very efficient numbers.

Fortunately my 99% design temp is ~8ºF and my area only sees 20hrs below zero per year, so this will work just fine for me.

This is true when it’s warm enough outside that you don’t need the heat pump anyway. When the temperature becomes cold, it’s a different story.

Heat pumps might make sense when you have a well-insulated house, like the kinds in Northern Europe, to trap heat and reduce the power required to heat the room. American houses in most places are far too drafty.

> Heat pumps might make sense when you have a well-insulated house, like the kinds in Northern Europe, to trap heat and reduce the power required to heat the room. American houses in most places are far too drafty.

If you need significant heating or cooling in a home, the first thing you need is to deal with is insulation, otherwise nothing you do is going to work well.

That’s true, but unless you are doing some kind of industrial work (including running very hot computers), you shouldn’t need significant heating or cooling in most of America.

The idea that indoors needs to be between 72° (F) and 75° at all times is unsustainable.

The other problem is that if you do decide to insulate the house, then you may end up needing to use A/C because the house can no longer be wind-cooled to an acceptable temperature, and you will need to update the heating system as traditional methods of heating (fireplace, stove, simple furnaces) depend on external air circulation.

> That’s true, but unless you are doing some kind of industrial work (including running very hot computers), you shouldn’t need significant heating or cooling in most of America.

> The idea that indoors needs to be between 72° (F) and 75° at all times is unsustainable.

The WHO recommends a general minimum household temperature of 64°F for all populations for health reasons, with a higher minimum for sensitive groups including children and the elderly; maximums are more regionally variant because acclimitization (which takes years) plays more of a factor in high-heat health risks, but even in extreme regions seems to top out at about 90°F for the general population.

Lots of the US spends lots of time significantly out of at least one end of that range (and, especially given that the top gets lower in colder regions, lots of the US spends lots of time significantly out on both ends of the scale.)

> The other problem is that if you do decide to insulate the house, then you may end up needing to use A/C because the house can no longer be wind-cooled to an acceptable temperature

If you have a house that is insulated well and also lacks doors and windows that can be opened, sure.

But... that presents other problems, too.

I’m talking about significant cooling and heating, which is what you mentioned. Set your thermostats to 66° F heating and 88° F cooling and compare your electricity gas usage to 72° F/75°F for the year.