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Sunsynk Whole House Backup Issue

Energy Storage

Last Post by Transparent 1 month ago

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LostandConfused

(@lostandconfused)

Active Member Member

Joined: 1 month ago

Posts: 4

Topic starter 12/01/2026 9:40 am

Hi, looking for help and advice as my installer is useless and Sunsynk have washed their hands of this.

I wanted a battery backup system, non solar, that we can run the house off during the day and then charge overnight on cheap rate. We have regular power cuts here and so I also wanted it to be a whole of house UPS so we dont notice the power cuts, whenever they happen.

The company I used installed the following system last month - Sunsynk 5kw hybrid inverter, 3 x 5kw batteries and Fox EPS. However, whenever the power cuts out, we’re noticing it as the FOX eps switches between grid and back up. The strange thing is that not everything cuts out when the grid goes - the clock on the oven, for example, never resets (where as previously with power cuts it would, it doesn’t have it’s own a battery back up in it either.)

The system isn’t making use of the Sunsynks UPS as I wanted whole of house backup and was told it wouldnt be possible without an EPS.

I’ve spoken to Fox about this and they say the cuts outs when switching are because their equipment isn’t compatible with Sunsynk. I’ve tried getting help from Sunsynk, but they’ve been useless, saying talk to the installer who planned the system and the installer…well, they’ve admitted there is a problem and want me to spend about £1000 to put it right.

Before I spend more money, I wanted to do my own research and find out the best option.

Does anyone have experience in this or how to put it right? If I got another Sunysnk inverter and put them in parallel, creating 10kw, would we then be better off using the UPS output for whole of house back up? Would getting an ATS switch just be the answer for the current setup? Should I just admit defeat and not have whole of house backup, splitting the consumer board and making use of the Sunsynk UPS?

Since the system was installed, peak is about 7KW at any one time.

I’m lost and would appreciate advice.

Thanks in advance.

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Transparent

(@transparent)

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Joined: 4 years ago

Posts: 2997

12/01/2026 12:04 pm

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

↑

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.

Save energy... recycle electrons!

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Batpred

(@batpred)

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Joined: 1 year ago

Posts: 602

12/01/2026 3:01 pm

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.

8kW Solis S6-EH1P8K-L-PLUS hybrid inverter; G99: 8kw export; 16kWh Seplos Fogstar battery; Ohme Home Pro EV charger; 100Amp head, HA lab on mini PC

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LostandConfused

(@lostandconfused)

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Joined: 1 month ago

Posts: 4

Topic starter 13/01/2026 9:54 am

@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?

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LostandConfused

(@lostandconfused)

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Joined: 1 month ago

Posts: 4

Topic starter 13/01/2026 9:55 am

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

👍

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Transparent

(@transparent)

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Joined: 4 years ago

Posts: 2997

13/01/2026 10:13 am

Posted by: @lostandconfused

↑

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!

Save energy... recycle electrons!

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Sheriff Fatman

(@sheriff-fatman)

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Joined: 10 months ago

Posts: 190

13/01/2026 11:56 am

@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.

130m2 4 bed detached house in West Yorkshire
10kW Mitsubishi Ecodan R290 Heat Pump - Installed June 2025
6.3kWp PV, 5kW Sunsynk Inverter, 3 x 5.3kWh Sunsynk Batteries
MyEnergi Zappi Charger for 1 EV (Ioniq5) and 1 PHEV (Outlander)
User of Havenwise (Full control Jun-Dec 2025, DHW only from early Dec)
Subscriber to MelPump App data via CN105 Dongle Kit

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Transparent

(@transparent)

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Posts: 2997

13/01/2026 12:04 pm

@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!

Save energy... recycle electrons!

👍

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