FoxESS Community - Owners & Installers Forum

oh dear, oh dear.....
An unexpected power cut tested out the new installation today...and I am not happy
The whole point of using this expensive system was to prevent interruption of the supply to the house

The website https://fox-ess.uk/eps-box-sp-backup-gateway/#eps-more makes a point of, and clearly states that it provides uninterrupted power, changes over instantly, seamlessly and without interruption

I quote:
Whole home backup for residential storage installations

The Fox ESS EPS-BOX-SP is a single-phase whole home backup gateway that provides uninterrupted power during grid outages. Designed for single-phase installations, it automatically switches between on-grid and off-grid modes to keep your essential (or all) household circuits running without interruption.
How it works

The EPS-BOX-SP sits between your Fox ESS inverter, utility supply, and household circuits.

Normal Operation – When the grid is active, power flows normally from your solar and grid supply.

Power Outage – The EPS-BOX-SP instantly disconnects from the grid and switches to your battery and solar supply, maintaining power to your home.

Grid Restoration – When the grid returns, it seamlessly reconnects and resumes standard operation.

...except that it doesnt....when the power cut happened the whole house went off, then a few seconds later (!!!!) the supply to the house resumed from the battery......when the power came back again the same thing happened

I later ran the test again this time using the supply isolator switch to give a clean break and exactly the same happened.....router died, NAS offline, CCTV offline, PC shutdown potentially with open files then went offline etc. etc

Even this says the switchover time is only 20mS

https://www.google.com/search?q=fox+kh7 ... fId=ChxjMe

This is exactly the scenario I was hoping to avoid!

Completely unrelated but it is now pitch dark but the panels are showing 240V albeit at zero current...

You are obviously upset, but what Inverter do you have, and what errors are showing on the screen?
If you have a small 3.68kW Inverter and 5kW load, it won't handle it.
If your battery SoC is not enough, it won't work either.
Then theres the chance they wired things up wrong.

If you can provide more details, then some help can be given hopefully.

Hi, indeed I am not happy.....
system comprises:
14 x Aiko Neostar 2p plus 475W panels (6.6 kWP)
Fox EP12 10.4kW usable battery (>40% SOC)
FOX KH7 7kw hybrid inverter
EN-EPS-BOX-SP gateway
Pretty much all perfectly matched as far as I can tell...

Same inverter as mine, should be able to handle 7kW EPS load... unless you was using more than that at the time.
Do you have fault codes on Inverter screen?

I would expect grid loss and Voltage warnings to be stored (and usually in the FoxCloud2.0 App you can view once running) on the Inverter screen/history.

No errors or messages
Load was maybe 3 or 400 watts no more

Sounds like you need to chase the installers down to check into this.
I added UPS to all the important items, don't want my NAS to turn off either

I have had three UPS running for years.....one on the PC & bband /wifi, one on the TV and AV system, one kn the CCTV etc.....as well as going solar I opted for the Fox gateway in order to get rid of the UPS and their expensive battery replacements
The specs clearly state NO break and elsewhere 20mS which is fine....not the several seconds I am getting

The wording in the page you link to is somewhat misleading, the EPS Box has a transfer time of "less than 3 seconds" according to the user manual https://theecosupermarket.co.uk/wp-cont ... 9VBc9c.pdf. As it uses an electromechanical changeover switch there is no way it can achieve a 20mS break.
The 20mS changeover is only mentioned in the inverter documentation for the EPS output.
So if I've read it correctly, the only way to achieve a 20mS maximum break time is to run the inverter as a UPS, directly feeding the load from the EPS output, with the EPS output set to operate in UPS mode.

yep, the inverter IS set to UPS mode

That makes the EPS output act as a UPS output BUT it is only connected to the load by the EPS box in the event of an incoming mains failure being detected by the changeover relay in the EPS box, hence the delay.

well that makes it totally pointless having ....
the whole point of the expensive whole home EPS we bought was to eliminate the breaks - exactly as the description would have you believe
we get quite a lot of power cuts and brown-outs hence the desire for the system we bought
I understand the requirement to have the inverter wired through the gateway to prevent it back-feeding to the grid in the event of a power cut but if what you are saying is correct then how can the house ever be proper UPS

If you check out Will's video, he tests a different switch box, and as you can see, it works how you want it to be.




Depending on what was said in your emails with the Installers, you may have a leg to stand on if you was told it would be instant.

@nrb501, thanks for the PDF, I did look and download last night, but my tired eyes missed the "3s" line in the PDF.

It does look like the Eco-ESS box will fulfil your needs - at least time delays and voltage parameters are configurable if required. It looks like the Fox EPS box just uses a cheap non-configurable automatic transfer switch.

Basic manual for the Eco-ESS box https://www.eco-ess.co.uk/wp-content/up ... ev-2.3.pdf

Hmmm...problem is i dont want to shell out more £££" to find it doesnt
The Fox one clearly and unambiguously states what it should do......

Have you checked emails to/from your installers to see how you/they have worded how your system will work?

If you asked for UPS like function and they didn't deliver, then they need to fix this.
If you made the mistake, sadly this could be on you utimately.

@ntb501....
thanks for the info
The manual states
When the mains grid supply returns to a healthy state the EPS-ATS will disconnect the emergency
power supply from the customers load before re-engaging the mains grid back to the customers
load after a 20s delay
am I reading this wrong or does that mean 20 sec with no power to the house ?

I still feel the Fox info is misleading and wrong...

davidwf15 wrote: ↑Wed Apr 01, 2026 11:27 pm When the mains grid supply returns to a healthy state the EPS-ATS will disconnect the emergency
power supply from the customers load before re-engaging the mains grid back to the customers
load after a 20s delay
am I reading this wrong or does that mean 20 sec with no power to the house ?

The system will monitor the incoming grid for a stable Voltage for 20s before it will switchover from EPS to GRID mode.
If the outside grid is not stable, the switchover won't happen.

You won't be without power for 20s.

I thought that would be the case
worst case scenario would entail buying 3 x small UPS's for the few seconds they would be needed.....I previously had two huge APC pure sine wave units, an SUA1000i running the TV/broadband/wifi and an SUA1500i for the PC, NAS and CCTV........I fitted them with external 38 and 45AH batteries which ran everything for several hours - certainly long enough for the power cuts (both for sale if anyone interested)
I guess for the few seconds they would be needed the cheaper and smaller stepped sine wave type would suffice
I still feel cheated and mis-sold at the wording on their websites though as it clearly and unambiguously states it provides a no break supply
I have logged a ticket with support to see what they have to say

Their UK website is very clear on what it can do, they have been cheeky when it comes to reality.

From reading around here and there, it seems there maybe a difference in markets, though I could be wrong. For example some get 3s switchover, others get 20ms or such.

It will be interesting to hear what Fox has to say on this matter.


I presume that you don't have any emails between you and the installer, as you haven't answered that question.

davidwf15 wrote: ↑Wed Apr 01, 2026 4:37 pm Hmmm...problem is i dont want to shell out more £££" to find it diesnt

The video from Will does show it working perfectly, but no sense shelling out £££ when you need to find out what Fox has to say first.

from the installer...wont name them (yet) but they are highly regarded in Ipswich and feature a green landscape

nothing is ever going to be done in a fraction of a second. That short delay is normal because your inverter is constantly monitoring the grid. When a power cut happens, it doesn’t switch instantly—it usually waits a fraction of a second to confirm the outage is real, not just a brief fluctuation or spike from the grid.

Once the inverter has confirmed there is a power outage, it disconnects from the grid and then prepares back up mode. This is always going to take a few seconds and usually a maximum of 5 seconds.

Fox have used the term ‘uninterrupted power’ very loosely, and it would be more accurate to say that your system has a swift switch over to backup so that you can continue with whichever activity you were doing, rather than having to stop completely until the power comes back on. It seamless and automatic in the sense that you do not have to manually instruct the system to go into backup mode.

There is no system out there that will complete the backup switchover in a fraction of a second, it is just not possible.

Fact remains that Fox are stating it is immediate and without interruption

Fox have replied....its not ambiguity it is clear mis selling and downright lies

I can see there is some ambiguity in the website content. What this is referencing is the automatic switchover - seamless in the sense there is no manual intervention.

A true UPS (zero-break) is not possible with equipment of this type - no manufacturer can offer this. A home battery system will detect outage, open the contactor, form grid and then resynchronise - so there is a short delay This is something we communicate to our installers during training so that they are informed when selling to customers - we demo the switch over with lights showing the 1-2 second delay. This is also referenced of page 1 of the product manual. The assumption would be that this is a known limitation of the technology, but we have instructed to web team to change the wording to avoid confusion like this.

For IT loads to have true UPS, a small plug-in UPS for critical devices is usually installed to bridge the short delay.

Definitely porkie pies, would recommend you print the webpage to PDF before they make adjustments.

I am sure that my own eyes, and Will's posted video do proove otherwise that it can be nearly instantaneous.
You are going to have to change the box over as it's not fit for your needs, or fit some small UPS's back to your kit again.

Depends on the lesser of two evils in your case.

Your installer clearly doesn't understand how no-break supply systems are meant to work. Using the appropriate devices it is perfectly feasible to achieve a switching time within 20ms, UPSs have been doing this for decades.
The biggest problem, and you can see this on Will's video at 6:26 (the light flicks off momentarily), for an off-line UPS (which the system with the Eco-ESS unit is) is when the incoming mains fails there is a finite time for the failure to be detected, a check that the back-up supply is present and then the change over to the back-up supply to take place, this leads to a short (~250ms) interruption when changing over to the standby supply, in this case the inverter EPS output. The change back can be almost instantaneous (~20ms) as both supplies are present.
With an "on-line" UPS the inverter is constantly supplying the load so there is no break when the incoming mains fails.

If the "UPS mode" for the inverter works as expected and the inverter is capable of supporting the full load on the EPS output then the load could be supplied more or less directly from the EPS output. Indeed the KH specs state <20ms switching time for the EPS output so this approach should work. You would also need an external switching arrangement so the inverter could be bypassed If necessary.
The user manual for the KH shows an example of EPS connection but with split "essential" and "non-essential" loads, although this would benefit from a bypass switch so the EPS loads could be supplied from mains if necessary. Note the manual also warns about keeping the peak EPS load below the inverter rated power to avoid the inverter shutting down and leaving you without power.

Thanks....we have monitored our power minute by minute for 18 months using a pi to count the meter flashes and it has never exceeded 6kW - hence the choice of the KH7 inverter and panels sizing
Technically we could just run the house on the inverter output but no doubt if ever there was a warranty claim it would be thrown out
Small UPS's it is then, have ordered 3 of these APC-Easy-UPS-900VA £69 from Amazon as I dont need any monitoring and they ae cheaper than the lower VA ones !

well well well... their website https://fox-ess.uk/eps-box-sp-backup-gateway/#eps-more has literally just been updated to add
"A short interruption (typically 2–3 seconds) occurs during transfer" ... but that wasnt the case when I bought it !
dirty ba****ds