Define cold.

I have an air-water heat pump as the only source of heat and hot water in my house. It takes heat from the air outside and dumps it into a 200 litre water tank. I’d guesstimate that 80% of the homes in my neighborhood have the same setup.

Temperatures are going to be around -10°C for the next few days. When the silver falls below -15°C a resistive element kicks in to help the pump. In my climate I won’t see more than a handful of those per year.

Nordic models scale down to -30°C before doing the same.