We had them when I lived in Montana, and we’d get down to -38C. They have a backup resistive heater built in. I’d supplement it with a little resistive heater, (mainly because I didn’t realize there was a resistive heat mode, lol), but even at its coldest, the air source heatpump was fine. This was in the US, but it was about $130 USD/month during the coldest months for about 800 square feet, decent insulation, double paned glass, ground floor apartment. That bill was for everything- heating, electric water heater, computer, lights, oven, washer/dryer/dishwasher. Summer power bills were about $60 for everything, and that was with the heat pump running cold air. Montana doesn’t even have the cheapest electricity. Suffice to say I was sad when my next apartment(s) did not have that, and I spent considerably more to heat and cool it, haha.

Aside from being remarkably less expensive due to their efficiency is the benefit that you don’t have to worry about natural gas /oil refills, fuel fires, and carbon monoxide from furnace issues. And depending on where you source your electricity from, you might be decoupled from the whims of OPEC/local production issues in your area.

I’d say, before you spend anything on your furnace or heatpump, I’d check your insulation and windows. Those go a long way in saving you money, regardless of outcome.