Depends on if its from ambient air temperature or if it’s hydronic (water or glycol). If air temperature drops to approximately 15/20° and the back up heat source kicks in, then it becomes costly. Heat pump air also is not like forced hot air. It usually feels cooler to the touch because your body temp is 97 to 99° and heat pump may blow out say 80°air temp. It will feel cool but you would be hot if room temp is 80.° Its all about increasing surrounding air temp. If air is lets say 0° and you want 70° then you have to increase air 70° but if it was hydronic and earth ground temp below frost line was 55° then to get 70° air temp you only have to raise temp by 15° to reach comfortable level of 70.° It takes alot more energy to increase air by 70° than it does to raise it 15° to achieve the same comfort level. A heat pump is just an air conditioner in reverse, keep filters clean and get serviced on regular basis in the long run it will benefit you. Everthing with regular maintenance works better lasts longer, from heater to cars to airplanes. Nj state certified HVAC/R Tech. EPA Cert. Universal & IMACA Cert. Since 1985 30yrs serving So.Jersey
The heat pump can generate heat all the way down to -640 degrees, with that said this will not keep you warm. The heat pump (depending on model and proper set up) can be very effective down to around 30 degrees. Of course this depends on the individual. The idea is to not run the electric part of the heating system, but this will be impossible in some climates. All you can do is keep the system cleaned and serviced regularly to maintain proper operation.
Its usually around 35 degrees. If you go with a heat pump that is a High efficiency variable speed compressor you can take that down further to the teens.
You would have to determine the balance point of the system to know for sure. Typically a heat pump can handle around 32 degrees then some sort of supplemental heat would have to be used.