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.

Not an expert, but apparently the latest ASHPs still have a CoP of around 2 at –20°C and unity at –30°C, which is pretty much the extreme low for the less-sparsely populated parts of the country. Ground source HPs are of course better in that respect but the initial investment is larger. Apparently GSHP is the most popular mode of heating in newly built single-family homes.

Thank you! In that case, they are very much worth it. In Romania, with the current absurd gas prices, even resistive heating is cheaper when powered by the national hydroelectric supplier.