Wiring faults, firmware issues, and hardware failures can all prevent EPS from working despite correct settings. A remote diagnostic session can identify the cause and advise on the fix.
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Important: EPS mode typically powers a dedicated backup circuit — not the whole house. If your installer didn't wire a backup circuit, EPS may be working correctly but not powering anything useful. If the whole house went dark, this may be expected behaviour — check whether a backup circuit was created during installation.
Check these in order — starting with the most common.
EPS mode is often disabled by default or was never configured. Check the inverter settings in the app or portal. This is the most common reason — and the easiest to fix.
Most systems require a minimum battery charge (typically 10–20%) before EPS will engage. If the battery was nearly empty when the cut occurred, EPS was inhibited. Check the portal's historical SoC data around the time of the outage.
Not all solar battery inverters have EPS capability. Basic hybrid inverters disconnect from everything during a grid outage as required by G98/G99. Check the inverter's datasheet or manual for EPS/backup specification.
EPS only powers what is connected to the inverter's backup output. If the installer didn't wire a dedicated backup circuit from the inverter to a backup consumer unit or sockets, EPS has nothing to power.
A firmware update may have reset EPS configuration. Check whether EPS is still enabled in the current settings — some updates revert to factory defaults.
Less common. The EPS relay or transfer switch in the inverter has failed. EPS settings appear correct but the hardware doesn't switch. Requires engineer investigation.
Work through these in order.
Find your inverter model name from the label on the unit. Check the manufacturer's datasheet for "EPS", "backup", or "off-grid" capability. GivEnergy hybrid inverters include EPS. Sunsynk and Growatt hybrid models include EPS. Basic string inverters without battery integration do not. If your inverter doesn't support EPS, backup power requires a hardware upgrade.
On Fox ESS, EPS settings are for your installer or a qualified electrician: see Fox ESS EPS not working.
Open your inverter's app or monitoring portal and find the EPS or backup power settings. Confirm EPS is enabled. On GivEnergy, this is in the System Mode settings — EPS must be explicitly turned on. Save and confirm the setting has persisted.
Find the minimum SoC for EPS in the settings — usually between 10% and 30%. Then check the portal's historical data to see what the battery charge was at the time of the last power cut. If the battery was below the minimum, EPS was correctly inhibited. Consider raising the charge target or lowering the EPS minimum SoC threshold.
Ask your installer (or check your installation documentation) whether a backup circuit was wired from the inverter's EPS output to any part of your consumer unit or to dedicated sockets. If no backup wiring was done, EPS has nowhere to supply power to — this is an installation gap that requires an electrician to add a backup circuit.
On Fox ESS, skip this step: don't switch off your mains to test EPS, because testing it is for your installer or a qualified electrician. See Fox ESS EPS not working.
With EPS enabled and the battery at a good charge level, you can test the backup function by briefly switching the AC isolator off and watching whether the backup circuit stays live. Wait 2–3 seconds after switching off before checking — the transfer time is typically under 100ms (about a second on GivEnergy hybrid and AC-coupled systems), but allow for it.
Only do this if you are comfortable with isolator switches and understand what circuits are and are not on the backup output. Do not do this if the inverter was recently indicating a fault.
If EPS is enabled, the battery was charged, and a backup circuit is wired — but backup still failed — there may be a wiring issue, relay fault, or firmware problem. A remote session helps pinpoint whether an engineer needs to attend.
Due to unprecedented demand we’re currently only carrying out on-site work in Yorkshire, and replies are taking up to 7 days. Remote diagnostics continue for the whole UK as normal.
Every message is read and answered, so please don’t send it twice; that only slows the queue.
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