Our heat pump (older model, not sure how old it is since it was in the house when we bought it) works pretty well down to -15. After that it's better to shut it

Our heat pump (older model, not sure how old it is since it was in the house when we bought it) works pretty well down to -15. After that it's better to shut it down, save the electricity, and use the good old wood heaters that have been installed since the house was built in the 50's.