Welcome to the forums Jack, so this changes throughout the year – air source heat pumps need to work less hard if the temperature of the air outside is higher so are far more efficient, so in the middle of summer for example you may get a coefficient of performance (CoP) at 4.5 – maybe even 5 – this means for every 1 unit of electricity it produces 4.5 units of useful heat.
The problem is when the temperature is colder (and you happen to also want the heating on) the efficiency drops right down – sometimes to less than 2.
What the industry have produced is a number known as the Seasonal Performance Factor which gives an annualised view of their efficiency. Therefore for an air source heat pump you are looking at an SPF of closer to 3.3. If your heat pump achieves this, you should be in pretty good shape!
Does anyone know what the top of the range efficiency is for an air source heat pump? Been getting lots of spurious salesmen giving me a whole range of figures from 2.5 to 5 times efficient. Not sure what to believe. Any insight would be helpful.