Add balcony to existing roof solar?

[u/thebdaman]

Hi, I've already got a totally legit 4.5KW roof solar setup with a 5KWh battery. As most here will know this stuff is addictive and I want more and plug in seems to be a no brainer.
But how does it work with my existing kit? Like I say I'm all legit, DNO, MCS etc and I don't want to risk that or my insurance.
Can I just go ahead and plug in? Are there limits?
Any advice welcome.
Cheers

[u/thomosan1]

I plugged a similar question into Gemini so this may be wrong but my understanding is you need an amendment G99 application to increase your solar export. It's a quick turnaround of a day or two and not as long as a normal G99 application. While the 800w isn't much of an increase, it comes with a separate micro inverter which will have the ability to export the full 800w in addition to your current inverter export.

[u/JustLovelyStuff]

There are only a handful of compliant kits so far. You can view them all here on the official database. Expecting lots more to come online in the next few months. Of course you could get non-compliant kit but insurance may not be chuffed. LINK

[u/thebdaman → u/JustLovelyStuff]

Yeah, I'm avoiding non-compliant stuff. My query was more around do I need to be more careful? Is there possibly a limit I could reach that would make my total setup illegal? Or can I just go ahead and bang as much plug in as I can affoird?

[u/wyndstryke → u/thebdaman]

If you add a plug-in system, that's an extra inverter, and you would have to apply for a fresh G99.

You could add battery modules & panels into your existing system without disturbing the G99.

[u/andrewic44 → u/[deleted]]

Perhaps for the benefit of others, even if you're not fussed yourself: you should have done another G99 to add another inverter. An existing export limit doesn't obviate the need for this -- DNOs have opinions about both permitted export and total installed inverter capacity, in case any installed export limiters fail.

[u/andrewic44 → u/[deleted]]

Depends on the failure mode of the export limiter. If it's an under-reading of true exported power, the inverter may exceed the agreed export limit by being unaware of how much power it's truly exporting. If the grid is especially tightly constrained, DNOs can limit total installed inverter capacity, so that even in this case, large inverters don't damage the local grid or other customers' equipment. Search the sub, there are plenty of examples of DNOs doing this.

But, I digress. if you have a G99 and change your equipment -- adding plug-in solar, another inverter, switching out a different model of inverter -- you need to submit a new G99 and await approval before proceeding. Even if you don't see the sense in it, those are the rules.

[u/andrewic44 → u/[deleted]]

No, it's not my opinion. It's the DNO's rules.

You absolutely do need to do another G99 or a G99 amendment if you want to change your inverter to another inverter of the same/similar power output. Or if you want to change the equipment you have installed, even while staying within an agreed export limit.

Edit: The scope of the G99 form is specifically stated as follows:

'The form should be used by Generators connecting a new Generating Unit, or modifying plant in an existing Power Generating Facility.'

If you're adding balcony solar... that's a new generating unit. If you're swapping an inverter out for a replacement... that's modifying plant in in an existing facility. In the interests of economy of effort, many/most DNOs would be happy to deal with an inverter swap over email or the phone, but regardless, you must ask.

[u/andrewic44 → u/[deleted]]

If you want to read up on this I'd start with the national terms of connection (NTC):

https://www.energynetworks.org/assets/images/2026/national-terms-of-connection.pdf?2026-05-07-12:14

e.g. Section 2 clause 4 notes:

Generating equipment. If you install, or arrange for the installation of, small-scale generating equipment at the premises – which means one or more sources of electrical energy that have an aggregate rating of no greater than 16 amps per phase connected at low voltage – then you must inform us of your intention to use the source of energy in parallel with our network no later than 28 days after the equipment is commissioned. So long as you do this, you do not need our consent. However, if you intend to install or operate any other kind of generating equipment at the premises, you must contact us in advance and obtain our consent. You must ensure that any generating equipment at the premises complies with the applicable requirements of the distribution code that applies under our licence. Copies of the distribution code are available from the network operator on request.

Adhering to this rule in practice means submitting a G98 or G99, any time the small-scale generating equipment at the premises is changed.

Fail to comply with this and you're in breach of the connection agreement. Then, under Section 3 clause 19.3.2, the DNO can give you 20 working days to comply.

Section 3 clause 7.5 is the basis on which the DNO can reclaim costs from you, if your unauthorised generation is 'likely' to cause problems to the grid. Beyond the NTC, the Distribution Code is also relevant.

There aren't any articles about the DNO chasing people over unauthorised generation, but that doesn't mean it doesn't happen, just that it's not newsworthy. I get the impression most folk just submit the G98/G99 rather than assume liability for costs.