I want to understand what happens when it's too cold out. And just running in pure air sourced HP mode, without supplemental heat.

Does it keep running at 100% but produces no heat? Limited heat? Does the house get colder and colder until everyone turns into a popsicle?

Or does it only heat the house to 18c instead of 20c?

In a climate where the low is -10c, how well does it work?

I bought into the heat pump hype until I bought a house with a furnace. Up until then I lived in apartments with heat pumps. I was stunned about how much better it was than any place I've lived before. It was used for it taking forever to get warm and always feeling colder than the thermostat would indicate. With a furnace it got warm quick, and it truly felt warm once it reached temperature. My power+heat bill was significantly lower per sqft than my power bill in my apartment.

I've lived in the Midwest, the Mountain West, and the South. I experienced the shortcomings of heat pumps in every place.

This article, which I believe to be geared at convincing US readers that heat pumps are great, also does some things that are extremely disingenuous.

For one, most heat pumps in Norway are geothermal heat pumps. Those are extremely different units that are well known to both be more energy efficient and function at much lower temperatures than air source heat pumps that are typically pushed in the US. The example where they interview a guy with an air source heat pump seems like an almost intentional misdirection.

Second, the author uses a comparison to electric furnaces. That has been widely known for years to be hilariously inefficient. As such it's fairly rare to see in the US. The most common sources of heat in the US are air source heat pumps (in places like AZ and Texas), oil radiators, and gas furnaces. Depending on energy prices, these could be significantly cheaper depending on utility cost. I understand Norway has specific conditions that make oil and gas usage much less appealing but, again, this article is clearly targeted towards westerners.

I feel like this is EVs all over again. Heat pumps have a lot of potential. They will one day before the de facto standard almost everywhere. However they have serious shortcomings and the idea that they are ready to be a drop in replacement in the vast majority of cases is hopium.

EDIT: Since everyone is getting caught up over the word "efficient". Electric furnaces are hilariously expensive.

No that's just my shitty heat pump. Sigh...

So my question with heat pumps is more how much does humidity effect the efficiency? Where I live is high elevation, has cold winters, but the air is dry as fuck. Single digit humidity for a month wouldn't be unusual.

My understanding is that heat pumps work best with humidity since moving moisture is part of how the heat is produced. When does a reasonably priced heat pump start falling off in efficiency?