Marion from the People Powered Retrofit Team here. We’ve been working on exactly this issue, trying to help clients understand all the numbers when it comes to heat pump sizing. Too often we find that heat pumps are over sized, which is both costly to run, and inefficient. We also advocate a fabric-first approach to retrofit meaning we prioritise improvements to the building fabric (insulation, airtightness etc) to reduce the heating demand first before switching to a heat pump. I could go into this further, but this hinges on a few things including how large a system can be run on single-phase domestic power, and electricity pricing being 4x that of gas.
Our current approach is to get a heat pump survey from an independent surveyor which looks at the fabric of the building, the current space and water heating demand, and the potential RHI grant in order to do a cost-benefit comparison based on capital expenditure, estimated running costs, and carbon emissions over the 7-year period RHI would be payable.
We’ve been lucky that we’ve found a heat pump installer who agrees with the survey findings and will use this information to size a system, however it is unlikely that this will always be the case as I understand MCS accreditation sits with the installer, with no separate designer accreditation, therefore the installer is liable for the sizing of the system (hence why they will do a survey themselves, usually for £200-300 which often comes off the install bill). We’re hoping this will change in the future, but this is the current system we’re working within.
In terms of capital cost for the system, this will change based on the manufacturer relationships (as you’ve noted), and any changes/upgrades to the radiators/pipework etc (as noted by @paul_chandler).
Where abouts on the south coast are we talking? We’re very much Greater Manchester-based, but we do have links to similar organisations around the country so might be able to point you towards someone in your area.
Hope this goes some way to answering your question?
PS @paul_chandler we’re looking to get to know heat pump installers in Greater Manchester - would be good to talk