> Eyeballing this graph[2] that corresponds to breakeven outside temperature of <-20℃ and -8℃ respectively for 40℃ output (~-18℃ and 0℃ for 50℃).
Interesting perspective, thank you. Considering that the average winter temperature in Seattle (Washington) is around 3-4˚C [1], it is far over the breakeven.
It is even more true in my case: my system is is a water-water geothermal type. They do maintain maximum COP (4-5) and maximum power independently of the outside temperature. They are perfect for cold climate.
For the little side story: My heatpump has been installed in 1987 by the previous owner of the house and I believe he was truly a visionary.
Nobody was talking or even speaking about heat pump at that time. From his own words: people of the neighbourhood took him for a mad-man when he said in the 80s he would warm his house with the cold water from the river. Facts are that his system is still running 35 years later and provided a cheap, reliable and low carbon source of heating for over 3 decades.
[1]: https://weatherspark.com/s/913/3/Average-Winter-Weather-in-S...