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Diagnostic guide · Fox ESS

Fox ESS EPS not working — backup power diagnosis

Power cut but your Fox ESS inverter didn't kick in? The EPS output may not be wired or switched on, the battery may have been too low, or the backup circuit may be overloaded. This guide covers the common reasons Fox ESS EPS fails and what to check before calling an electrician.
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Tell us whether the EPS output is wired, the battery charge at the time of the power cut, and any alarm names in Fox Cloud. We go through your Fox Cloud settings and history remotely.

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Diagnosis

Step-by-step EPS backup diagnosis

Work through these in order. Step 1 comes first: without EPS wiring, nothing else here will give you backup.

1
Confirm with your installer that EPS is wired and switched on

Fox ESS hybrid, AC-coupled and all-in-one inverters have an EPS output, but wiring it is optional at installation, so not every system has backup. Ask your installer whether it is wired to the circuits you want backed up. Without opening anything, look in your handover pack or electrical certificate, and for a separate board labelled EPS or backup. If the EPS output is not wired, the inverter cannot supply backup power, whatever the settings in Fox Cloud. On the H1-G2, EPS is also off by default and is switched on in the inverter's own settings, normally by the installer at start-up. That setting and any EPS wiring are for your installer or a qualified electrician.

2
Check Min SOC and Min SOC (On Grid) settings

In Fox Cloud or on the inverter's screen, look for the two minimum charge settings. Fox ESS names them differently by version, so here is the power-cut setting first in each pair: the inverter menu has Min SoC and OnGrid Min SoC; Fox's July 2025 app manual has System Min SOC and Battery Reserve Capacity; Fox's February 2026 app manual (V2.1) has Backup Cut-off SOC and System Min SOC. The power-cut setting is the level at which the battery stops supplying the home in a power cut. The grid-connected setting sets how much charge is kept back while the grid is on, so a low value can leave little for a power cut. Fox ESS prints no recommended figure, so ask your installer what suits your system.

3
Consider Back up work mode to hold a battery reserve

Fox ESS describes the Back up work mode as designed for areas with unstable power grids: it reserves battery charge to keep power going during outages. If you have frequent power cuts, ask whether Back up mode suits your system. In Self-Use, the battery discharges freely during the evening to offset grid import, which means it may be empty when a power cut hits. In our view Back up mode gives up some daily savings, and it only helps if the EPS output is wired and the load is within its rating.

4
Check for overload on the EPS circuit

The EPS limit depends on the model: up to 6,000 VA on the H1-G2 and 5,000 VA or 6,000 VA on the first-generation H1, with a higher peak allowed for up to 60 seconds. Other models are listed below. If the load on the backup circuit stays over the limit, the inverter raises an Eps Over Load alarm. Our advice is no kettles, ovens or immersion heaters on EPS, and the same goes for electric showers: high-power appliances like these may exceed your model's rating. Keep to essentials such as lighting, fridge, router and a few sockets. Motors and fridges draw more when they start, so their starting power must also stay under the EPS maximum. Changing which circuits sit on EPS is an electrician's job.

5
If an RCD trips when EPS starts, call your installer

If an RCD trips every time EPS starts, don't keep resetting it. EPS wiring and earthing is an installer's job, not a Fox Cloud setting. Call your installer or a qualified electrician and tell them exactly when it trips.

6
Have EPS tested, and check for alarms afterwards

Don't test EPS by switching off your mains: testing it is a job for your installer or an electrician. In a power cut the inverter should switch to EPS by itself and run the connected circuits from the battery. Fox ESS prints the switch-over time for the H1 and H1-G2 as under 20 milliseconds on the inverter's own EPS output (its newest H1-G2 datasheet says under 10), and under 3 seconds on the H1-G2 through the EPS BOX-SP. After a cut, look in Fox Cloud for alarm names such as Eps Over Load and Eps Relay Fault, and see the Fox ESS fault code index for what they mean. Don't count on Fox Cloud during the cut itself, because the datalogger and your router need power. If EPS still fails, contact STS for a remote diagnostic: the cause may be a firmware fault, a wiring error or an inverter setting that needs your installer.

EPS limits by Fox ESS model

H1-G2
Up to 6,000 VA
By size, from 3,000 VA in the UK (2,500 VA is the smallest size on Fox's global datasheet), with a higher peak for up to 60 seconds.
H1 first generation
5,000 VA or 6,000 VA
KH
Up to 10,500 VA
Single-phase, from 7,000 VA by size. Fox ESS UK lists 7 to 10 kW: 10,500 VA is the global KH10.5.
EVO 10
Up to 10,000 VA
By size. Whole-home backup needs the EPS BOX-SP.
H3 and AC3
Up to 12,000 VA
Three-phase. At most a third of the rating on any one phase off-grid.
H3 Smart
Up to 15,000 VA
Three-phase, by size. At most a third of the rating on any one phase off-grid.

With the grid off, the load must stay under the limit, even with whole-house backup. The figure for your exact model is in its datasheet on the Fox ESS documents page.

Fox ESS EPS backup: what you need to know

What EPS does

EPS stands for Emergency Power Supply. On a Fox ESS inverter, the EPS output gives battery-backed power to the circuits wired to it during a power cut. It is not a no-break supply. EPS wiring is optional at installation, so a home can have an inverter and battery and no backup at all.

Whole-house backup

Without a changeover, only the circuits wired to the EPS output get backup. For the whole house, Fox ESS offers the EPS BOX-SP on the single-phase H1-G2, KH and EVO, and the EPS BOX PRO-HC-G2 on three-phase. Each is bought separately. The first-generation H1 and the AC-coupled AC1 can't take the EPS BOX-SP. The EPS BOX-SP changes over automatically, and it doesn't show as a device in the app. The inverter's EPS limit still applies.

Solar in a power cut

On a hybrid inverter, the panels keep charging the battery during a power cut. Fox's AC-coupled AC1 and AC3 have no PV input of their own.

What else Fox ESS lists

Fox ESS also lists an Eps Relay Fault alarm, and inductive loads, such as motors and compressors, whose start-up power must stay below the EPS maximum. Those are for your installer or Fox ESS. For what each alarm means, see the Fox ESS fault code index.

Related Fox ESS guides

Backup power and guides

FAQ

Questions about Fox ESS EPS backup

Backup needs the EPS output wired, EPS switched on in the inverter, charge in the battery and a load within the inverter's limit. EPS wiring is optional at installation, so a home can have a battery and no backup at all. If the output is wired, check the Min SOC (On Grid) setting: if the battery can drain to a low level during normal evening use, there may be nothing left when a power cut hits.
You don't set it up yourself: EPS wiring is a job for a qualified electrician or your installer. The EPS output is wired to the circuits you want backed up, directly, through a changeover switch, or through one of Fox's whole-home boxes: the EPS BOX-SP on the single-phase H1-G2, KH and EVO, and the EPS BOX PRO-HC-G2 on three-phase. EPS is off by default on the H1-G2 and is switched on in the inverter's own settings, normally by the installer at start-up. To have it added, see EPS installation; for help with the reserve settings, book a remote diagnostic.
Min SOC is the level at which the battery stops supplying the home in a power cut. Min SOC (On Grid) is how much charge is kept back as a backup reserve while the grid is on, so a low value can leave little for a power cut. Fox ESS names these differently in the inverter menu and in each version of the app. It prints no recommended figure for either, so ask your installer what suits your system.
It depends on the wiring and the total load. Without a changeover, EPS only supplies the circuits wired to it. For a whole house you need a changeover switch or one of Fox's whole-home boxes (the EPS BOX-SP on the single-phase H1-G2, KH and EVO, and the EPS BOX PRO-HC-G2 on three-phase), and the EVO 10 needs the EPS BOX-SP for whole-home backup. Even with whole-house backup, the inverter's EPS limit still applies, so high-power appliances like electric showers and ovens may exceed it and are best kept off the backup circuit.
An RCD that trips when EPS starts needs someone on site to find the cause. EPS wiring and earthing is an installer's job and not a setting in Fox Cloud, so don't change anything yourself or keep resetting the RCD. Call your installer or a qualified electrician and tell them exactly when it trips. If you're not sure who to call, contact STS.
The remote diagnostic is free: with your permission we go through your Fox Cloud settings and history, through your own login or after an account transfer (£95). If it's a setting we can put right remotely, such as the reserve level or Back up mode, it's £125, and only if we fix it. Physical work is quoted in writing before we start: EPS wiring is from £495, a dedicated backup circuit from £995 and a full-house changeover from £1,495.
A settings-only fix, such as the reserve level, is usually same-day in a single remote session once we can see your Fox Cloud account. Wiring the EPS output or fitting a changeover switch is a booked visit, quoted in writing before we start. The free diagnostic tells you up front which of the two you're dealing with.
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Get help if your Fox ESS EPS backup still isn't working

Tell us whether the EPS output is wired, the battery charge when the power cut happened, and any alarm names in Fox Cloud. We cover all Fox ESS models, and with your permission we work through your login or an account transfer. Independent from Fox ESS and your installer.

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