Posted by: @ronin92

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Can I just put in an AC-coupled second inverter+battery with 1-2 panels (I have space for that) to avoid VAT?  Will the DNO permit that?  It will be grid-side of the existing generation meter to stay-FiT compliant.

Do you feel up to doodling your intended system diagram and posting it here please @ronin92 ?

You can literally do a pencil sketch on the back of an envelope and take a smartphone photo!
I have the facility to redraw that with relevant graphic icons if we need it to aid our discussion.

You will certainly need consent from your DNO.
Which region are you in?

In adding another string-inverter and a storage battery, your house will effectively have three devices with possible export capability to the grid:

• existing solar inverter
• new PV inverter (which will export if the battery is full)
• the storage battery itself (depending on the mechanism it uses to create the 50Hz sine-wave output)

The DNO would evaluate those three devices, then look at the method used for export-limitation, and issue a G99 approval letter if they're happy.

It costs nothing to make an application for G99.
This can also be done by the householder rather than a professional installer.
So if your strategy is rejected, you can always amend what you've proposed and then resubmit.

The majority of G99 applications are handled using a Fast-Track process.

The Electrical Networks Association issues the following guidance for you to check if G99 Fast-track applies to your site

If that seems like a mess of terminology, then please say so and I'll expand it until it makes sense!

I've deliberately avoided any comment of costs, viability, FIT-payments etc. at this stage.

@transparent Thanks.

I'm envisaging a system something like that below. I want to add batteries to a MSC-approved solar installation registered on FiT scheme.  This installation has a nominal 4 kWp output but I've never seen >2.6 kW because of its E-W configuration.

The only reason my proposed new installation has panel(s) included is to avoid VAT.  I'm wondering if it looks like the kind of thing that gets approved.

I've been speaking to prospective installers. 

One prefers me to replace the existing inverter with a hybrid inverter+battery as it saves getting DNO approval.  I am reluctant to do that as battery round trip losses from panels and grid will be deducted from my FiT generation.  Another tells me their lawyers say VAT is not applicable for batteries if you already have a solar installation which is news to me!

Posted by: @ronin92

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One prefers me to replace the existing inverter with a hybrid inverter+battery as it saves getting DNO approval. 

That's because you and he are only considering that new inverter also having grid export capability.

Have a look at this posting from me yesterday.
It has diagrams showing the two main approaches to inverters coupled to batteries.
The description is still valid, even though that topic was started by a member who does not have PV solar panels.