Posted by: @jamespa

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a) even if the battery inverter is set so that it does not discharge when this would lead to export, I still need G99, which will need to specify both inverters, because the DNO assumes it might export.  This might limit the max output of the battery inverter even though its not intended to export

I have just been through this. I had an electrician connect a battery to an inverter to my consumer unit (and I had previously had the CU replaced to improve safety).

I am not familiar with combined battery+inverter, my preference was to have a set that works, I chose low voltage lifepo4 as I understood it was the best value. I made the mistake to get a battery kit, there are now ready made 16kwh batteries starting at £1000.

I chose a solis hybrid inverter that provides 4ms switch from power failure, essentially it is fast enough that no device notices it. I did some tests. I switched the consumer unit off, all ok, the inverter made a few clicks and all continued as normal. I then switched it back to grid, again no issues, no flicking, as the inverter kept handling the load with the battery. It took some time to defer to use grid power. I think these timings are mandatory for approval.    

I was not planning to export, but when I contacted my DNO (UK Power Networks) to let them know I had an inverter, they said they did not need to know unless I would be exporting. They "offered" 5kw export and said "just fill in the details in the initial G99 form as if G98". In short, right now I have an export MPAN.. for 8kw.. All without any MCS overhead.

As I underspent, we are now considering investing the spare funds and getting PV a bit earlier than we thought.

The regs may be a bit more restrictive than what UKPN allows.. So it may depend on what your DNO says and it may depend on your local grid as well. 

Posted by: @jamespa

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b) If the inverter does support islanding, there must be a suitable cut out device |(presbetween the supply and any part of the internal wiring that will be energised during a power cut, to ensure that nothing is fed back to the grid in this circumstance

Have I got this correct?  Im not trying to 'design' the system, just understand so I am an informed customer!

Officially you may, and there is apparently only one brand of inverter in the list that does not require a separate device.

@transparent kindly provided an explanation of the oscillators the inverters have to support islanding, etc.  

Still, my experience was that UKPN was happy to accept the Solis G100 capability. I contacted them after the system was up and running (I always made sure it was set to not export). When filling in the G99 commissioning form, provided the inverter is type approved, much of it does not need to be tested on your installation. it is "out of the box". Very little work involved, it seemed. They were very helpful. 

Posted by: @jamespa

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Also, has anyone looked seriously at sodium ion batteries.  They seem to be obtainable and are, at least prima facie (but who knows what happens under the hood), more ethical in production and lower fire risk.

That may be a longer route.. If you search for sodium, some suggestions were made, but I think nobody ordered it, or if they did, I do not recall a report of the experience. 

G99 needed for ac coupled battery. DNO will assume it can export unless it is type tested G100 and installed how the DNO likes to be export proof. Limiting export via software settings is not considered inherently safe or reliable.

PV inverter + battery inverter together count as combined generation capacity.

If you want it to power the house during a grid outage, it must have a certified anti-islanding mechanism. Must comply with Engineering Recommendation G98/G99 and ENA anti-islanding requirements.

Sodium ion batteries are heavy, LFP cells are now so cheap the shipping is the majority of the price. So sodium has to "compete" against that. Sodium is dirt cheap, but you'll never avoid the weight. The temp tolerance is moot in domestic storage application where its being constantly cycled.  They have lower power density and a wider, annoying, voltage range, which means inverters will need to change for them. 

To make clear, my Solis inverter has anti islanding. But because it has a couple of separate ports that it can serve independently from the grid, those can continue to work based on PV or battery. So only the grid port looses power to prevent islanding.  

@swwils 

Hi It well worth getting the G99 as it opens up "options"

I have the give energy AIO and "gateway" and yes i to had to get a G99 giving me 9.64kw export.

the AIO can discharge at 6kw which works for us.

the Gateway will go into  "island Mode" if the grid is down all Automatically.

also when the AIO is support the home any solar will continue to charge or run the house, without the need for the grid.

A 2 box solution, just works ! and is tidy !

I see the Powerwall 3 can do a higher discharge  and they do an equivalent to a "gateway"

Regarding Sodium I think in a year that be more options but your need more space for the same capacity.

I to will expand my storage with sodium when it more of a option..

@batpred The EPS output can be tricky to do to current regs. The old "rcd socket for type 2 only!" doesn't really cut it anymore.

Posted by: @swwils

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@batpred The EPS output can be tricky to do to current regs. The old "rcd socket for type 2 only!" doesn't really cut it anymore.

I have the export MPAN, what are you talking about?

This is how it is working, before exporting..

Posted by: @rob-heatpump-bob

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Regarding Sodium I think in a year that be more options but your need more space for the same capacity.

I to will expand my storage with sodium when it more of a option..

I am curious... which inverter would you go with for sodium batteries? 

Posted by: @rob-heatpump-bob

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I have the give energy AIO and "gateway" and yes i to had to get a G99 giving me 9.64kw export.

We submitted our papers for SEG, so hopefully we will soon unleash our inverter with export. Keen to see what Solis AI can do with that. 

@rob-heatpump-bob I am curious if your DNO accept 9.64kW on a single phase circuit?