@chickenbig I tend to agree with your interpretation of VAT Notice 708/6...
... but it needs to state that explicitly. That what VAT Notices are for!

I've run VAT-registered SME companies for the past 40 years.
The nature of that work means that I buy in products which have different levels of VAT applied to them by the supplier.
I'm well used to those concepts.

The present reading of 708/6 is ambiguous. For example

The [0%-rated VAT] relief applies to the services of installing energy-saving materials in residential accommodation. For example, where a customer employs a business to install energy-saving materials that the customer purchased directly from a retailer.

So HMRC have made some attempt to cover aspects of the householder making the purchase of the Energy Saving Materials (EMS) themselves.

Posted by: @chickenbig

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The VAT notice applies to installers only.

How to account for VAT if you’re a contractor or subcontractor installing energy-saving materials and grant-funded heating equipment.

That's the title of the VAT Notice.
It doesn't have legislative authority!

As Sir Humphrey Appleby explained to Jim Hacker:
If we want the public to believe that we're doing something which we don't actually want to do, then it's best to put it in the title...

Posted by: @chickenbig

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Whether the installation of an extra battery counts as an energy saving measure will no doubt be clarified

@toodles is right that the provision to include storage batteries has been announced by HMG in early December ('23).
That is explained in numerous 'advice briefings' from major UK accountancy firms(!)
but not (yet) reflected in an updated VAT Notice or a dated Government Directive.

The Government seems to have civil servants scrabbling around to re-edit the provisions into previous announcements, but doing so isn't aiding clarity. Such amendments and alterations aren't themselves dated or printed in a different font.

For example, here's text which has been altered/inserted on 11th Dec'23. The document concerns a Government Response to a public Call For Evidence which was opened after the Spring 2022 Financial Statement from the Chancellor:

The government recognises the integral role that electrical battery storage can play in reducing households’ reliance on the grid, both when retrofitted to a qualifying ESM and when used as a standalone technology connected to the grid.

This technology already qualifies for VAT relief when installed alongside a qualifying ESM, as it is ancillary to a zero-rated supply. The government agrees with respondents, though, that this technology meets all of the stated objectives for this VAT relief in its own right, and will therefore bring electrical battery storage within the scope of the relief. This means that the technology will now also qualify for VAT relief when either retrofitted to a qualifying ESM, or installed as a standalone technology connected to the grid.

@transparent I found out about the 5% VAT rate that could be applied to the purchase of a second Tesla Powerwall by wrinklies like myself before my installer was aware; their accounts dept. had to go away and confirm what I had told them before they could give a quote for supply and installation last year. Government departments appear to grind exceeding slow, even if (in the case of HMRC at least) they grind exceedingly fine! Regards, Toodles.