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Diagnostic guide · Fox ESS
Fox ESS EPS not working — backup power diagnosis
Power cut but your H1 inverter didn't kick in? The EPS port may not be wired, the battery may have been too low, or the backup circuit is overloading. This guide covers every reason Fox ESS EPS fails and what to check before calling an electrician.
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EPS didn't activate?
Tell us whether the EPS port is wired, the battery SOC at the time of the power cut, and any alarm codes in Fox Cloud. We diagnose EPS faults remotely.
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Andrew Palmer · May 2026
Safety first: EPS diagnosis involves your home's electrical supply. Do not open the consumer unit or modify any wiring yourself. The steps below cover settings and checks you can do in Fox Cloud and at the inverter. Any wiring changes — including adding EPS circuits or neutral-earth bonding — must be carried out by a qualified electrician.
Diagnosis
Step-by-step EPS backup diagnosis
Work through these steps in order. Step 1 catches the majority of cases — the EPS port was never wired during installation.
1
Confirm the EPS port is physically wired
The H1 inverter has a dedicated EPS output port, but it is not always connected during installation. Many UK systems are fitted without the EPS circuit wired — the port is simply left unused. Check with your installer whether the EPS output is connected to a sub-board or changeover switch. If the EPS port is not wired, the inverter cannot supply backup power regardless of any settings in Fox Cloud. Adding EPS wiring after installation is possible but requires a qualified electrician.
2
Check Min SOC and Min SOC (On Grid) settings
In Fox Cloud, check the two minimum SOC settings. Min SOC is the absolute floor — the inverter will never discharge below this point, even during a power cut. Fox ESS recommends 10%. Min SOC (On Grid) is the minimum level during normal grid-connected use. If this is set to 10%, the battery routinely drains to near-empty during evening use, leaving almost nothing for EPS when a power cut occurs. Set Min SOC (On Grid) to 20–30% to ensure a meaningful backup reserve is always available.
3
Consider Backup work mode to hold battery reserve
The Backup work mode in Fox Cloud holds the battery at a high state of charge and only discharges to cover essential loads during a power cut. If you live in an area with frequent outages or need guaranteed backup, switch from Self-Use to Backup mode. 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. Backup mode sacrifices some daily savings but ensures the battery is always ready for EPS.
4
Check for overload on the EPS circuit
The H1 EPS output is limited to 3kW to 5kW depending on the model. If the total load on the backup circuit exceeds this, the inverter trips on overload and shuts down EPS output. High-draw appliances — kettles, electric ovens, immersion heaters, electric showers — will almost certainly exceed the EPS capacity. Reduce the load to essentials: lighting, fridge, router, and a few sockets. EPS should recover automatically once the excess load is removed and the grid fails again or a manual test is triggered.
5
Check neutral-earth bonding (RCD tripping on EPS)
The Fox ESS H1 does not create a neutral-earth bond internally when operating in EPS mode. Without this bond, RCDs on the backup sub-board may trip immediately when the grid drops — cutting power to the circuits you are trying to protect. The fix is an external N-E bonding relay that activates automatically when grid power is lost. This is an installation requirement, not a Fox Cloud setting. If your RCDs trip every time EPS activates, this is almost certainly the cause.
6
Test EPS with a simulated power cut
If wiring and settings look correct, test EPS by switching off the main grid supply at your consumer unit. The inverter should detect the grid loss within approximately one second and switch to EPS mode, powering the connected circuits from the battery. Check Fox Cloud for any alarm codes during the test. If EPS still fails to activate, contact STS for a remote diagnostic — the issue may be a firmware fault, a wiring error, or an inverter configuration that needs installer-level access to correct.
Fox ESS EPS backup — what you need to know
EPS stands for Emergency Power Supply. On the Fox ESS H1 hybrid inverter, the EPS port provides battery-backed power to selected circuits during a grid outage. The system detects grid loss and switches to battery power within approximately one second — fast enough for most electronics but not instantaneous. The EPS port must be physically wired to a sub-board or changeover switch during installation for backup to work. Many UK installations leave this port unconnected, which means the inverter and battery are present but the home has no backup capability at all.
For whole-house backup, a changeover switch such as the Fox ESS EPS Box transfers the entire property to inverter power when the grid fails. Without this, only circuits on the dedicated EPS sub-board receive backup. The H1 EPS output is rated between 3kW and 5kW, so high-draw loads must be excluded. The two most common problems STS diagnoses remotely are Min SOC (On Grid) set too low — leaving the battery empty before the power cut happens — and missing neutral-earth bonding, which causes RCDs to trip the moment EPS activates. Both are fixable without replacing any hardware.
Related Fox ESS guides
Fox ESS EPS backup — common questions
The most common reason is that the EPS port on the H1 inverter was never wired during installation. Many UK systems are fitted without the EPS circuit connected, leaving the inverter with no path to deliver backup power. If the port is wired, check the Min SOC (On Grid) setting — if the battery drains to low levels during normal evening use, there may be nothing left when a power cut hits.
EPS requires physical wiring from the inverter's EPS output port to a dedicated sub-board or changeover switch — this must be done by a qualified electrician. Once wired, EPS is enabled in Fox Cloud under inverter settings. Set Min SOC (On Grid) to 20–30% so the battery always holds a reserve. For whole-house backup, a Fox ESS EPS Box or third-party changeover switch is needed. If you need help configuring EPS settings, book a remote diagnostic.
Min SOC is the absolute minimum the battery will ever discharge to — even during a power cut. Fox ESS recommends 10%. Min SOC (On Grid) is the minimum during normal grid-connected use, and this is the key setting for backup reserve. Setting it to 30% means the battery stops discharging at 30% during normal operation but can draw down to the Min SOC level during an actual power cut — giving you the full range between the two values for emergency use.
It depends on the wiring and total load. By default, the EPS port only supplies a dedicated sub-board with selected circuits. For whole-house backup, you need a changeover switch such as the Fox ESS EPS Box. Even with whole-house backup, the H1 EPS output is limited to 3–5kW, so high-draw appliances like electric showers and ovens will overload the system and should be excluded from the backup circuit.
The H1 inverter does not create a neutral-earth bond internally when in EPS mode. Without this bond, RCDs cannot detect fault current correctly and trip immediately. The solution is an external N-E bonding relay that activates when grid power is lost — this is a wiring modification by a qualified electrician. It's a known requirement for Fox ESS EPS installations, not a fault. If you're unsure whether your installation has this, contact STS for a remote assessment.
The remote diagnostic is free — we check your Fox Cloud settings and tell you whether EPS just needs enabling and a Min SOC reserve set, or whether the EPS port was never wired. If it's a settings fix (enabling EPS, setting Min SOC On Grid, switching to Backup mode) we can usually do it remotely with installer-level access for a flat £75, and only if it works. The physical side — wiring the EPS port, fitting a changeover or EPS box, or adding the neutral-earth bonding relay — is an electrician job from £245, quoted after the diagnostic.
A settings-only fix — enabling EPS and setting the Min SOC reserve — is usually same-day in a single remote session and takes effect immediately. If the EPS port needs wiring, a changeover switch fitting, or an N-E bonding relay, that's a scheduled electrician visit, typically a half-day. The free diagnostic tells you up front which of the two you're dealing with.
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EPS still not activating?
Tell us whether the EPS port is wired, the battery SOC when the power cut happened, and any alarm codes in Fox Cloud. We diagnose Fox ESS backup faults remotely using installer-level portal access. Independent from Fox ESS and your installer.
- Not affiliated with Fox ESS
- No fix, no fee
- On-site visits available across the UK
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