I'm curious as to how low a temperature your heat pump will operate. I live in New England and replaced a whole-house air conditioner with a heat pump, but the heat pump works only to 35F. Much colder than that, and an auxiliary electric heater kicked in. The first Winter cost me about $800 over my gas-fired forced hot water heating system. I had the contractor disable the electric heat in the Spring and rewire the thermostats to start the (high efficiency) furnace when the outdoor temp got too low.

Mine can go until -25c they say: https://www.nibe.eu/en-eu/products/heat-pumps/air-water-heat...

We don't ever get those temps so I should be fine.

My biggest issue is not cold but mist. I live near a river in a valley and have underestimated how much mist hurts performance around ~1c outside.

It needs to defrost often, because of the high moisture content in the outside air where I live.

But it also has a normal, resistive heating 9kW backup. But for financial reasons this is considered 'emergency only'.

Keep in mind that heat pumps have a limit how much they can pump (it also depends on temp., there is less heat in 35F air). If your house is not well insulated, at a lower temperature it would be loosing more energy and eventually it would reach the threshold where it's performance is not enough to keep up.

That depends on the refrigerant, the new Mitsubishis are effective at that and lower temperatures.

We got a heat pump in BC Canada, it’s rated down to -30C.

We also got solar, our entire power bill (all heating, cooking, lights, computers, etc) is $500 for the year. Best decision ever.