I profoundly disagree with the dismissive people on this thread in all but a few very extreme edge cases. There are heatpumps rated for down to -30°C. If you live somewhere where it gets colder than that, then yes, you'll need a backup system. In all other cases it's just a matter of getting a heatpump that can handle your local climate (I'd argue it's a good idea to get one that can handle at least a couple of degrees below the coldest recorded temperature in your area, just to be safe)
I realise it might sound hollow to say that I don't think you need a backup, given that I myself actually do have a backup in the form of a fireplace. Well, my house is old, even by Swedish standards. A letter I found in a jar under the floor when I was redoing the ground insulation a couple of years back claims the house was built in 1840. I have of course updated the fireplace to be compliant with modern fire safety standards, but the original construction predates heatpumps by some margin. If not for that I probably wouldn't have had a backup. I might have gotten a second pump to help with my chilly office, but that's really more about my house being too big for the pump I have than it is about heatpumps not being able to "hack it".
I realise it might sound hollow to say that I don't think you need a backup, given that I myself actually do have a backup in the form of a fireplace. Well, my house is old, even by Swedish standards. A letter I found in a jar under the floor when I was redoing the ground insulation a couple of years back claims the house was built in 1840. I have of course updated the fireplace to be compliant with modern fire safety standards, but the original construction predates heatpumps by some margin. If not for that I probably wouldn't have had a backup. I might have gotten a second pump to help with my chilly office, but that's really more about my house being too big for the pump I have than it is about heatpumps not being able to "hack it".