NC7 alternative

curioustony · 14-06-2025 03:21PM #1

Was wondering if this is legal with an NC6, and if it would work in some situations as a viable alternative to getting an NC7.

Basic idea is that

the house load runs exclusively on a paired set of 6 or 8Kw inverters (not grid tied), using backup port for scheduled charging (looks like an appliance to the grid). Providing some level of redundancy.
the 5Kw grid tied inverter is exporting 20 or 21 hours/day (assuming enough battery), and can participate in charging when prices are low/negative. Changeover switch to keep using the panels attached to the 5Kw
Battery is shared by all three inverters

Thoughts?

graememk · 14-06-2025 03:55PM

Lets assume your talking about sunsynks/deye as thats the only ones I know that parallel easily.

They dont charge from the load/backup port, Possibly you could use the Gen/Aux input on them to do that.

Unfortunately, That is still an inverter connected to the grid, its grid tied. So that would still not be legit.

If not looking to charge from a higher rate overnight, extra panels could be connected to the battery via charge controllers instead of inverters. How often would you actually need more than 5kW of discharge power in your house?

curioustony · 14-06-2025 04:58PM

I was thinking of the GEN port on the deye alright. I think Victron also has this capability. My bad for using the 'backup' word. It's ambiguous.

I figured that it would in this configuration be the same as a rectifier to boost cheap rate charging.

5kW is regularly hit cooking dinner, but very rare for it to stay there for long.

I was trying to make sure that the two inverters were completely decoupled, no back feed, from the grid and have some redundancy in case of failure as the whole house load is always from these. Basically an off grid house, with an NC6 backup, and FIT maximisation.

So it all hinges on the definitions of inverter and connection. I hoped an inverter connection was bidirectional so the GEN port would not count. And I guess an inverter is not the same as a rectifier.

graememk · 14-06-2025 05:33PM

https://www.boards.ie/discussion/comment/123543777#Comment_123543777

I dont think it would export on the Gen port. It only connects when its looking to charge from the gen port OR you have it configured for microinverters

but internally, the Grid, Gen and Load all connect to the same bus bar. the inverter is connected - well into a grey area there.

curioustony · 14-06-2025 05:44PM

https://www.boards.ie/discussion/comment/123543903#Comment_123543903

Fair enough, too good to be true then. Had never considered the inverter internals. Pity. Seemed like an obvious way to have a large system with an NC6. Thanks!

curioustony · 15-06-2025 08:24AM

So I could not help myself and with a little help from a LLM (ask and verify), if seems for the deye at least, that there is no common busbar. Isolation is achieved using L & N relays. This is part of the island mode story. It pointed me to the manual, which I thought was inconclusive (implied in the notes on the wiring diagram), and the certificate of conformance, which I thought was better (explicit mention of relays), but as a non-expert, not definitive. It does make sense though as generators are not likely to appreciate back-feeding...

https://www.deyeinverter.com/deyeinverter/2023/12/13/certificate_ireland%2Bi.s.en50549-1.pdf

https://www.deyeinverter.com/deyeinverter/2024/02/03/instructions_sun-3.6-6k-sg03lp1-eu_240203_en.pdf

Apart from being gray, the only negatives I could come up with were potential waste if the battery was too small, and how fast the inverters would respond to load. I know that with my system, we need to be careful what we turn on at the same time during grid outages.

graememk · 15-06-2025 09:53AM

https://www.boards.ie/discussion/comment/123545409#Comment_123545409

I've seen it referenced in tear downs (and also verified somewhat by testing)

Each output is relay controlled (I actually think redundant relays) onto this internal busbar or voltage rail. The common rail is behind the relays.

The module that does the battery DC-AC is also connected to this rail, as is the PV inverter.

Yes I don't think generators would appreciate being back fed! I dare say when charging from a "generator" it will only connect itself when needing to charge from it. (After synchronising itself with the generators phase)

In the eyes of ESBN it would count as an inverter connected to the grid. But would they be able to know? Very unlikely.

Teardown of a Deye :

https://youtu.be/n0_cTg36A2Q

curioustony · 15-06-2025 11:59AM

https://www.boards.ie/discussion/comment/123545555#Comment_123545555

I've never watched one of those. He seemed quite unclear on the GEN isolation, apart from the fact that it is switched with redundant relays.

The component bashing was fun, and potentially underlines the need for redundancy.

I have no plans to do this. Herself indoors would not be happy. Nor do I want to risk an arguement with ESBN. I just cannot see why this would be a bad or dangerous setup.

graememk · 15-06-2025 12:44PM

https://www.boards.ie/discussion/comment/123545834#Comment_123545834

There's a difference between, bad, dangerous, and ESBN rules.

There's nothing bad or unsafe if it's done properly.

Somewhat along the lines why the NC6 doesn't allow export limiting rather it's a limit on inverter size. The NC6 is an inform and fit, installers just send it in, and fit the inverter. Don't have to wait for approval or further checks.

NC7 alternative

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