Many thanks for the reply, I think I understand you.
New build houses here often have Mechanical Ventilation with Heat Recovery (MVHR) , there are however appearing on the market a number of Exhaust Air heat pumps directly connected / integrated with a DHW cylinder. e.g. ESP Ecocent, CTC EcoWater, Giona often installed as a Continuous Mechanical Extract Ventilation system.
None are approved for any government support, so that skews peoples requirements. - A single or twin heat pump that does Space Heating and or DHW attracts government support. However exhaust air heat pumps aren’t (yet) supported in this way
All of them appear to offer a better CoP because the air input temperature is (usually) higher than the outdoor ambient, (A15 / A20 as you say) in theory they also offer a greater energy saving advantage than MVHR, however I have yet to do the math as the heated air used by the above DHW cylinder systems are using air from the dwelling heated by another source.
You are correct in that most heat pumps have a built in heating element (auxiliary heater), however we have to design that out of our systems for them to qualify for the government grants, so we are looking for higher efficiency if possible.
There are a couple of off the shelf PV diverter controllers that can switch heat pumps on to heat the DHW cylinder and ensure they don’t cycle to frequently (max 6 times / hour is a top end target) I’ve not looked at seeing if the OEM Solar PV diverter could be configured to do this.
There are lots of products available including the OEM Solar PV diverter that can run the Immersion heater in the DHW cylinder (some are better than others )- however they are only 100% efficient compared to the 350% efficiency of a heat pump