How solar battery backup power works EPS, UPS, island mode and anti-islanding explained
Most people assume their solar battery will power the house during a power cut. Most of the time, it won't — unless the system has been specifically configured for it. This guide explains how backup power actually works, what the acronyms mean, and what determines whether your lights stay on when the grid goes down.
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When the grid fails, every grid-connected solar inverter in the UK is legally required to shut down within milliseconds. This is true regardless of whether your battery is fully charged. The reason is a safety rule called anti-islanding — and understanding it is the key to understanding how backup power works.
Grid fails → inverter detects loss of grid voltage → anti-islanding activates within 200ms → inverter shuts down completely → battery sits idle → your house has no power until the grid returns. This is the default state of most UK solar battery systems.
Grid fails → inverter detects loss of grid voltage → anti-islanding disconnects from grid → EPS circuit activates within 20ms → inverter enters island mode → battery powers your protected circuits → solar panels can continue charging the battery. Your backed-up circuits stay on.
The difference between these two scenarios is not the battery — it's the wiring and configuration. The battery is there in both cases. What makes backup work is a combination of a compatible hybrid inverter, EPS-specific wiring between the inverter and your protected circuits, and the EPS mode being enabled in the inverter software.
Anti-islanding — why your system shuts down
Anti-islanding is a safety mechanism mandated by G98/G99 regulations in the UK. It exists to protect people — specifically, electricity network engineers who work on the power lines during an outage.
When the grid goes down, engineers assume the lines are dead and work on them accordingly. If your inverter continued to push electricity back into the grid during the outage, those lines would still be energised — and anyone touching them could be electrocuted. This is called "islanding" because your house would become an electrical island, generating power independently while still connected to the wider network.
To prevent this, every grid-connected inverter sold in the UK must include anti-islanding protection. The inverter continuously monitors grid voltage and frequency. The moment either falls outside the permitted range (voltage below ~207V or above ~253V, frequency outside 47.5–52Hz), the inverter disconnects from the grid within 200 milliseconds and stops generating.
EPS vs UPS — what the acronyms mean
These two terms get used interchangeably, but they mean different things. The distinction matters because it affects what appliances can survive a power cut without interruption.
Transfer time: ~20 milliseconds (varies by inverter brand). The grid fails, the inverter detects the loss, disconnects from the grid, and switches to island mode. There is a brief interruption of roughly 20ms.
What survives: Lights, fridges, freezers, gas boilers, internet routers, phone chargers, TVs — anything that can tolerate a 20ms blip. Most appliances do not even register the interruption.
What may not survive: Desktop computers (may restart), NAS drives (may corrupt if writing data), some medical equipment (check with manufacturer), certain alarm systems.
Cost: Included in the inverter capability. Wiring installation from £795.
Transfer time: Zero. True UPS systems use a double-conversion topology — power flows through the battery at all times, so there is no switchover gap when the grid fails.
What survives: Everything, including sensitive electronics. Desktop computers, servers, medical equipment, and any device that cannot tolerate even a momentary interruption.
Typical use: Data centres, hospitals, medical equipment at home, professional recording studios. Rarely needed for domestic solar systems.
Cost: Significantly more than EPS. A standalone UPS unit sized for a few critical devices starts at £200–£500. Whole-house UPS is specialist territory.
Island mode — how the inverter powers your home without the grid
When EPS activates, the inverter enters what is called "island mode". It stops being a grid-following device and becomes a grid-forming device — creating its own AC supply from the battery, independent of the grid.
In normal operation, a grid-tied inverter synchronises its output to the grid's voltage (230V) and frequency (50Hz). It follows the grid. In island mode, the inverter generates its own 230V 50Hz supply using the battery as its energy source and an internal oscillator for frequency reference. It becomes a standalone power source.
This is possible because hybrid inverters have a dedicated EPS output circuit that is physically isolated from the grid connection. When island mode activates, a relay disconnects the EPS output from the grid side and connects it to the inverter's standalone output. This physical disconnection is what makes it safe — there is no electrical path from the inverter's island supply back to the grid.
What your battery can and cannot power during an outage
Battery backup is not unlimited. The two constraints are your inverter's EPS output rating (typically 3–5kW continuous) and your battery capacity. Understanding these limits prevents the disappointment of a battery that runs flat after two hours.
Estimates assume 7.6 kWh usable (80% of 9.5 kWh) with no solar input. During daytime with good solar conditions, panels recharge the battery while it powers your home — extending duration significantly. Actual results depend on battery health, ambient temperature, and real load.
LED lighting, fridge, freezer, internet router, phone/laptop chargers, TV, gas boiler (pump and controls only), alarm system, doorbell camera.
Kettle (1-minute burst is fine), microwave (short use), washing machine (high spin cycle draws 2kW+), tumble dryer. Monitor battery level and avoid running simultaneously.
Electric shower (8kW — exceeds most inverter EPS ratings), electric oven/hob (3–4kW sustained), immersion heater (3kW), heat pump (2–4kW continuous). These will drain the battery rapidly or overload the EPS output.
How to check if your system has backup power
Not sure whether your solar battery will power your home during an outage? Work through these checks.
Most hybrid inverters from GivEnergy, Sunsynk, Growatt, SolaX, Fox ESS, Lux, and Sigenergy support EPS. Standard string inverters (SolarEdge single-phase, Fronius non-hybrid, SMA Sunny Boy) do not — they shut down completely when the grid fails. If you are unsure, check the inverter model against the manufacturer's spec sheet or look for an "EPS" or "Backup" option in the inverter settings menu.
Log in to your monitoring portal or inverter app. Navigate to system mode or backup settings. On many systems, EPS is disabled by default and must be explicitly turned on. If EPS mode shows as enabled but you've never tested it during an actual outage, it may be enabled in software but the wiring may not be in place — which means it still won't work.
The battery reserve (or minimum SoC) determines how much charge is kept in reserve for backup. If this is 0%, the battery may fully discharge during normal daily use, leaving nothing when the grid fails. A reserve of 20–30% is typical for EPS-enabled systems. This setting is in the battery or energy management section of your monitoring portal.
EPS only covers circuits that are physically wired to the inverter's EPS output terminals. A single EPS socket covers one outlet. A dedicated EPS sub-board covers the circuits on that board. A full-house changeover covers everything. If you are unsure which circuits are protected, check your consumer unit — an EPS sub-board is a separate small board, usually near the main consumer unit, labelled "EPS" or "Battery Backup". See the EPS installation guide for the four configuration types.
The only way to be certain is to simulate a power cut. With the battery charged above reserve level, turn off the main incoming supply at your consumer unit (the large switch at the top). Your EPS-protected circuits should remain powered within a few seconds. If everything goes dark, either EPS is not configured, the wiring is not in place, or the neutral-earth bond is missing.
If your backup does not work during the test, see no backup during power cut for full diagnosis steps.
Which inverters support backup power?
EPS capability depends on the inverter type, not the battery. The battery stores energy — the inverter determines whether and how that energy can be used during an outage.
GivEnergy — All-in-One, AC Coupled, Gen 1/2/3 hybrid
Sunsynk — All hybrid models (3.6kW, 5kW, 8kW, 12kW)
Growatt — SPH, SPA series
Fox ESS — H1, H3 hybrid models
SolaX — X1/X3 Hybrid G4
Lux Power — All hybrid models
Sigenergy — SigenStor hybrid
Alpha ESS — SMILE series
Huawei — SUN2000 with LUNA2000 (SPS mode, single-phase only)
SolarEdge — Single-phase models without backup interface. The SolarEdge Home Hub with backup interface does support it, but the standard SE3000–SE6000 range does not.
Fronius — Primo, Symo (non-hybrid models). The Fronius Gen24 Plus with BYD battery does support backup.
Solis — String-only models (S6 mini). The Solis S6-EH1P hybrid does support EPS.
Enphase — Microinverter systems do not support EPS without an additional IQ Battery system and Enphase System Controller.
Aurora/Power-One — All legacy models (discontinued)
Eversolar — All models (discontinued)
Frequently asked questions
Only if your system has EPS mode enabled and the correct wiring installed. Most UK solar battery systems are grid-tied and shut down completely during a power cut, even with a fully charged battery. You need a hybrid inverter with EPS capability, EPS mode enabled in settings, a battery reserve above zero, and the protected circuits physically wired to the EPS output. Without all four, the battery will not power anything during an outage.
EPS (Emergency Power Supply) switches to battery power within about 20 milliseconds of a grid failure. Most appliances survive this brief interruption without issue, but sensitive electronics like desktop computers may restart. UPS (Uninterruptible Power Supply) provides zero-interruption transfer using double-conversion topology. Most domestic solar systems offer EPS, not true UPS. If you need zero-interruption for specific devices, a small standalone UPS downstream of the EPS circuit is the standard solution.
This is the most common misunderstanding about solar batteries. Grid-tied inverters are legally required to disconnect from the grid within milliseconds of a power cut (anti-islanding, mandated by G98/G99). Without EPS wiring and configuration, the inverter shuts down — the battery is there but cannot be used. To enable backup, you need EPS mode enabled in the inverter settings and the correct wiring between the inverter's EPS terminals and your protected circuits.
It depends on battery capacity, reserve level, and load. A 9.5 kWh battery with 20% reserve has roughly 7.6 kWh usable for backup. Running essentials only (lights, fridge, router — about 500W average) gives around 15 hours. Adding a gas boiler reduces this to about 10 hours. Running high-draw appliances like an electric shower (8kW) drains the battery in under an hour. During daytime, solar panels continue to charge the battery in island mode, extending the duration significantly.
Yes, if your inverter supports it in island mode. Most hybrid inverters with EPS capability continue to harvest solar energy while operating in island mode. The panels charge the battery and the battery powers your circuits simultaneously. A daytime power cut with good solar conditions can be sustained almost indefinitely. However, the inverter limits output to match the connected load — surplus energy is not exported to the grid while islanded.
Anti-islanding is a safety mechanism required by G98/G99 regulations. It forces grid-connected inverters to disconnect within milliseconds of a power cut, preventing the inverter from back-feeding electricity into the grid while engineers work on the lines. This is why your system shuts down by default. EPS works around anti-islanding by creating a physically isolated circuit — the inverter safely powers your home without any connection to the grid.
Related guides and resources
Compare EPS sockets, dedicated circuits, manual and automatic changeover. From £795 (socket) to £795 (dedicated circuit).
Battery didn't kick in? Diagnosis steps for EPS configuration, wiring, and neutral-earth bond issues.
Battery chemistry, charge cycles, BMS architecture, and self-consumption economics.
DC/AC conversion, grid synchronisation, and the difference between string and hybrid inverters.
The grid connection standards that mandate anti-islanding and define export limits.
Safe shutdown and restart procedure — AC and DC isolation sequence for all brands.
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