> The heat pump model is based on linear interpolation in a performance map retrieved from manufacturer data (Daikin Europe N.V., 2006).
And that model is half as good as the modern one I linked:
> The nominal coefficient of performance (COP) is 3.17 at 2/35ºC and 2.44 at 2/45ºC test conditions (i.e. air/water temperature) for full load operation.
And even then it gets a COP of 2 at -15C. So modern systems getting to at least 4 at -15C seems likely. This claims there are low ambient mini splits that maintain 100% efficiency down to -15C:
That should cover most places but for really cold climates ground source heat pumps seem ideal. From what I've seen there are simple solutions where a single vertical hole is drilled with common well drilling machines and a single tube that has the loop inside is driven down. Makes it easier to retrofit and implement in small properties.
Keeping 5x over the entire winter includes some pretty cold nights. And ground source systems can improve even this by quite a bit.