Hi @lostandconfused - you're in luck.
And you're quite right to do some research here for yourself.

This issue is currently being discussed in the more general sense on the topic Recommended Home Battery Inverters.

Have a read through that Topic first because it introduces the overall concepts rather than diving straight into a technical problem.

The answer to one of your questions above is addressed in my response here in that topic

Posted by: @transparent

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The basic concept for deciding the number and size of of inverters and batteries is this:

If you need more power to run the home, then add inverters - that increases the kilowatts

If you need your stored energy to last longer, then add battery capacity - that increases the kilowatt-hours

I've got hands-on experience of installing hybrid off-grid battery storage, and I'm currently part-way through switching from three Growatt inverters
to three Sunsynk 5kW units.

Based on what you've just written above I don't think you should've needed a Fox EPS,
but I'd like the option to change my mind as you tell us more of what you want to achieve! 😀 

It would help if you (re-)posted your questions to the end of that other topic.
That will enable others to read the solutions in future.

Once I see you do that, I can close down this separate Topic.

Hi @lostandconfused , I agree that topic/thread is the most appropriate, it started with a potential upgrade of a simple solar inverter system to enable ups/backup/islanding. And it is a shame your installer washed their hands off it. 

It may help if you are able to post a simple diagram so we can better understand what you have. 

We have a fully automatic system based on a Solis S6 hybrid inverter and a single battery. I was my own installer.

Our system makes all likely powercuts seamless, and it exceeded my expectations. I am reproducing the diagram below. 

@transparent Thanks v much! With your sunsynk, will you be doing whole of house via the UPS or splitting the consumer unit for just essentials?

@batpred I'll do my best to put together a simple diagram and get back to you, thanks.

Posted by: @lostandconfused

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With your sunsynk, will you be doing whole of house via the UPS or splitting the consumer unit for just essentials?

I have options to do both.

I'm an innovator, for which the storage battery facility is one of the 'tools' in development work.

I currently have four mains Consumer Units, but that's because I sometimes need to change my mind as to which circuits are supplied from where.
There are also two solar-panel 'switching and protection' boards,
and a number of low-voltage distribution 'boards'.  😎 

Ie I run a significant number of home devices directly from the 52v battery, without power going back through the inverters.

Since you're particularly interested in off-grid operation, then I can directly address that issue...
... but preferably over on the other topic, where the fundamentals of storage batteries are already discussed!

@transparent 

Slightly tangential query, but one that you can perhaps assist with as someone with both extensive knowledge of the electricity/grid structure and, specifically, the Sunsynk inverters and associated software.

My 5kW Sunsynk inverter is limited to G98 export limits, and the DNO have advised that there's no realistic prospect of increasing this via G99 due to the number of solar PV systems in proximity in our area (a suburb within Leeds), so 3.68kW is as good as it will get.

I'm trying to find a way within the settings to limit the grid export to 3.68kW, but to retain the 5kW inverter capability for other internal activity, such as solar PV charging to the batteries and general discharging of the batteries for consumption in the house.  I believe that the Sunsynk inverters are capable of doing this (please correct me if I'm mistaken) but I can't apply this in the settings.  Within the 'system mode' section of the settings, there's an Inverter Power Limiter setting that logically could be set to 5000W.  Separately, within the Grid Settings section, there's a Export Power Limiter setting that logically has to be set to 3680W to prevent any risk of breaching the G98 limit.

As soon as I change the grid export setting, the overall inverter power limiter setting changes with it, and vice versa.  If I set the export to 3680W, and then change the overall inverter power limiter to 5000W, the export setting increases with it.

At present it looks like a software restriction to me and, if so, potentially something I could follow up with Sunsynk directly.  I don't have the technical knowledge to understand whether it's perhaps a more basic hardware restriction, and I'm trying to achieve something that isn't technically possible.  I doubt there's a better informed person available anywhere to advise whether or not this is the case.

Apologies for using the thread to add a separate query, but it seemed a logical place to do so.

@sheriff-fatman - could you please re-post that in the other Topic here, because we're going to archive this topic out of public view!

But I will say now that I don't (yet) know the answer to that question...
... although I'm pretty certain it will have been addressed in Synsynk's own online forum!