Backup EPS output

Dedicated backup circuits, transfer behaviour and load limits.

An EPS (emergency power supply) output is a connection on some battery and hybrid inverter systems. When it has been incorporated into a complete backup arrangement, it can supply designated circuits while the public grid is unavailable. Product documents may use terms such as EPS, backup output or secure supply.

Having the terminal does not mean the home is already backed up. The output must be designed into dedicated circuits or a compliant changeover arrangement, with isolation, earthing and protective devices that work in both grid-connected and island modes.

What happens during an outage

When the grid fails, a backup arrangement separates the maintained circuits from the public network before supplying them in island mode. The changeover may be built into a wider gateway or use separate switching and isolation equipment.

Normal operation and transfer behaviour vary by product. Some EPS circuits receive grid power while it is available and switch to the battery during an outage. Other systems use a separate transfer device. Transfer time belongs to the exact equipment and installed arrangement, so an EPS label alone does not establish no-break or UPS performance.

A home battery backup is therefore not automatically a substitute for equipment-specific continuity protection.

Dedicated loads or whole-home changeover

A dedicated EPS consumer unit supplies selected circuits. This keeps the backup demand within the inverter and battery limits and leaves other circuits disconnected during island operation.

A whole-home gateway or changeover device can make more circuits available, but it does not increase inverter power. Some systems use load control or load shedding to keep demand within the available output.

Power, surge and energy are separate limits

Continuous backup power determines which loads can run together. Short-duration surge capability affects compressors, pumps and motors. Battery energy and reserve state of charge determine how long they can run.

The lowest relevant limit may come from the inverter, battery BMS, battery temperature, state of charge, phase arrangement or control mode. A large battery can still have a modest EPS output, and a high-power inverter cannot provide long runtime from an almost empty battery.

Solar may extend an outage only if the particular system can operate its PV input while islanded and has enough battery or controllable load to stabilise it. Ordinary grid-tied PV elsewhere in the property will normally remain disconnected.

Earthing and fault protection

Protection that works while connected to the grid cannot simply be assumed to work in island mode. The complete design must provide the necessary isolation, earthing and protection in every operating mode. The correct arrangement depends on the supply earthing system, the selected equipment and its instructions.

Reserve, restart and owner expectations

Backup also depends on the energy available when the outage begins. A reserve setting can retain some battery capacity for this purpose, but black start, restart after overload and PV operation in island mode are product-specific. The system documentation should state how these functions behave.

Commissioning and handover

Commissioning should include a simulated loss and restoration of the grid supply, not only operation while the grid is present. The checks cover the changeover arrangement, island-mode protection, maintained circuits and safe reconnection.

The handover information should identify the maintained circuits, power limits, operating procedure and isolation arrangements. Labels should make clear that circuits can remain live from a local source after the main grid supply is isolated.

Applies to

Battery

Last reviewed

22 Jul 2026