alt Hacker NewsSee [1] for the theory.
I couldn't find a model/calculator that would help visualise typical COP values for particular climatic conditions. However, you'll find in your travels that a COP of ~2-2.5 is typically achieved for ambient (outdoor) temperatures of -15oC (for air sourced heat pumps).
If 300L of water at 15oC is filled into a tank and needs to be heated to 60oC within 2 hours during ambient temperature of -15oC, you get very approximately (no thermal losses considered):
- An output energy need of approximately (4190300(60-15))/(60*120)=~8kW (56MJ/2h)
- An input electricity need of approximately (8/2.2)=~3.6kW
Instead of a resistive heating hot water unit requiring 16kWh to do this job, you could use a heat pump hot water unit requiring 7.2kWh, cutting electricity use in half.
And this is for arguably the most extreme use case for a heat pump hot water unit where it's "cold started" right at the coldest moment in Winter in cool-temperate climates (such as SE Australia). Think for example, arriving at a ski chalet and having to turn on the hot water unit before someone can take the first hot shower.
On a more typical day of the year, perhaps with overnight ambient temperature of 10-15oC, the COP would rise to ~4, equating to an electricity consumption of 2kWh to heat the 300L of water. A lot of units will be set to heat during the warmest part of the day, let's assume an ambient temperature of 25-30oC, where a COP of ~5-6 is more typically achieved. However, there are obviously diminishing returns for COP of 4 vs 5.
In arctic climates, heat pumps are still used, but with a ground or aquifer source rather than ambient air source.[2]
[1] https://en.wikipedia.org/wiki/Coefficient_of_performance