Whilst waiting for HMRC's reply, my initial view is that : Is the contractor installing an energy saving system?
In your scenario, is the contractor installing energy saving materials (ESM) or are they simply removing existing ESM, doing something to your roof and then putting back the ESM and whilst doing so, adding a few extra panels?
If the panels have to be removed to repair a leaking roof, the primary purpose here is to fix the leak/replace the tiles, to facilitate that the contractor has to remove the panels, fix the leak and put the panels back, so you need to look at why the panels are being removed in the first place, because that is the motivation/driving factor here.
I don't automatically agree with the suggestion that you have to buy an inverter in order to qualify, the inverter is but one component of what is a multi-component system (panels, framework, inverter, possible boarding of the loft, battery, etc) but it is a bit more nuanced than that. The question is whether or not the project is the installing of energy saving materials or merely the removing of them to get access to the roof for some other reason/job?
If we assume the panels have been removed to say fix a leaky roof (remove panels, fix tiles/leak, put back panels), the job is clearly one of fixing a leak and the whole job would be standard rated, removing the panels is necessary to access the roof tiles, that's the only reason the panels are being removed. Putting the panels back is not re-installing an energy saving system, it was already installed beforehand.
In your scenario, you seem to be suggesting that you have extended your solar panel system by adding more panels at the same time as having some refurbishment work done, lets assume those works are to fix a leaking roof/get the roof repaired. Let's say you had a 10 panel system before and then after they were removed to fix the leak, the contractor is installing 5 additional panels so that you end up with a 15 panel system.
There is a view that the 10 original panels were already there, they were removed to fix a roof leak and if put back as was, then we'd accept the whole thing is standard rated as the contractor is not installing an energy saving material, they are fixing the roof. So where the contractor adds panels as part of putting things back after fixing the leaks, there is an argument that the 5 additional panels could potentially be zero rated, but the rest of the works are standard rated, but then adding panels to an existing set-up is that not just a supply of goods which are added to an existing system?
What about installing a new air source heat pump and new radiators at the same time? The whole project is zero rated as the new radiators are required for the heat pump to work properly, but if you later extend the property and add a couple more radiators, those radiators are not zero rated because they are just radiators, even though they form part of an overall heating system which is energy saving.
It could be the contractor is taking a similar view with your project, they are simply adding on a few extra panels (like my example of extending the existing heat pump system by adding an extra radiator or two) and adding a few extra panels to what was already there is not "installing an energy saving system", the energy saving system is already there....and by that logic, this is may be how the contractor is talking about you needing a new inverter to qualify - not saying they are correct - its certainly not stated in the guidance - but trust my explanation shows how they possibly arrived at that conclusion.
Depends on the facts, you don't detail what the refurbishment works are but trust you can see that the question is much more nuanced than might first seem.
HMRC guidance, section 2.9.2 explains the issue https://www.gov.uk/guidance/vat-on-energy-saving-materials-and-heating-equipment-notice-7086#installations-of-energy-saving-materials and also check out 2.9.3 and 2.9.4 with .9.4 making the point that if you have a house extension which includes fitting thermostatic valves to all radiators in the house, the overarching project is the extension and the whole project is standard rated (even the radiator valves), but if after the extension has finished you then get a contractor in to install new radiator valves, that job alone can be zero rated.
HMRC may struggle to give you a definitive answer - nobody knows the precise details of your project, you could write to HMRC and detail the project, with plans and before/after and HMRC may then take a view, other than that, the guidance is the main source of decision making and it'll be between you and the contractor to negotiate....it does not sound like the contractor is trying to pull a fast one, sounds like they are trying to firm up the job as being zero rated by adding the inverter which then looks much better on paper than just adding 5 panels to an existing system (which sounds like para 2.93/2.9.4 and comes across as just extending an existing system). In the same para's HMRC talk of a scenario where the extension is done first and then the thermostatic radiator valves are added later as a new project and in that scenario the valves can be zero rated, whereas if done at same time as extension, the extension is the dominant supply.
HMRC are consulting with the industry and experts with a view to amending and updating their existing guidance (the notice I link to above) with a view to further clarification and scenarios such as installing an additional battery storage to an existing system which already has battery storage and other little quirks which have cropped up since the zero rate was extended to cover solar panels, etc.
