A standard grid-tied solar and battery system does not keep your lights on in a power cut. The moment the mains supply fails, the inverter disconnects itself and your panels and battery sit idle until the grid returns. That surprises many owners, because a battery is sold on the days it saves you money, not on the one situation where it does nothing. Whether your solar battery works in a power cut comes down to a few things being present and correctly wired, and the type of backup you paid for decides how much of the house stays on.
So do solar panels work in a power cut? Only if the system was built for it. To keep even a fridge and a few lights running you need a backup-capable inverter or battery system, a battery with charge left in it, safe isolation from the grid, and the specific circuits you care about wired to the backup output. Miss any one of those and the system stays dark. I own and run a Sunsynk 3.6 ECCO hybrid and a Fogstar battery, but I have not wired or commissioned its backup output, so the earthing and wiring detail below comes from the manual and the connection rules rather than from a blackout I have sat through.
Why a normal solar system shuts off in a power cut
Grid-tied inverters are required to detect loss of mains and disconnect, so they never feed electricity into cables that network engineers may be working on. In Great Britain the connection rules are Engineering Recommendation G98 for smaller systems and G99 for larger ones, and both require this anti-islanding protection. Northern Ireland applies the equivalent G98/NI and G99/NI. A compliant system therefore shuts down in a power cut even with a full battery and bright sun on the roof.
This is a safety feature, not a fault. The inverter must stop supplying the network the moment it detects that the mains has gone, and it does so quickly. Without that rule, a solar system could keep a dead section of street cable live and put a lineworker at risk. The shutdown is the system doing exactly what the grid code tells it to.
Getting power during an outage therefore means creating a small, deliberately isolated island inside your own home that is fully separated from the grid. That is a different job from ordinary grid-tied operation, and it needs specific hardware to do it safely.
How to check whether your own system can do it
If you already own solar and a battery, you can usually tell in a few minutes whether backup was ever built in. Look for a backup or EPS output on the inverter, check whether any circuits are actually wired to it, and confirm a reserve is set aside in the app. If none of that is present, your system will shut down in a power cut like any other grid-tied install.
- Find your inverter model and open its manual. A hybrid inverter with a terminal or mode labelled EPS, backup, or off-grid can run an island. A plain grid-tied string inverter has no backup terminals at all. EPS stands for Emergency Power Supply.
- Look for a backed-up circuit. A commissioned backup usually shows up as a second small consumer unit, or a labelled socket, fed from the inverter’s backup output. If every circuit runs straight off your normal consumer unit, nothing is wired for backup.
- Check the earthing. Safe island operation needs its own earth reference, because in most UK homes the earth normally arrives through the grid supply. On the Sunsynk ECCO the manual makes an earth rod a condition of running the backup output. No dedicated earth arrangement usually means no commissioned backup.
- Check the reserve setting in your app. Many hybrids let you hold a minimum state of charge back for outages, set as a percentage floor. Without it, normal daily use can empty the battery before a power cut arrives.
- Ask your installer the six questions below, in writing. They settle what the hardware will actually do before you rely on it.
Popular UK hybrids from Sunsynk, GivEnergy, Solax, Fox ESS and SolarEdge, along with the Tesla Powerwall, all offer a backup mode. None of them provides backup until that output has been wired, earthed and commissioned. The brand on the box does not tell you whether yours is ready. The wiring does.
What your system needs to keep working
For a solar battery to work in a power cut, several parts have to be present at once: a backup-capable inverter or battery system with a backup output (often labelled EPS, backup, or off-grid), a battery holding usable charge, safe isolation from the grid, and the circuits you want to keep alive wired to that backup output with a suitable island-mode earth arrangement. Many batteries are installed grid-tied with the backup output left unwired or switched off, which is why owners discover the gap only during the next outage.
- A backup-capable inverter or battery system. A plain grid-tied string inverter has no backup terminals. A hybrid inverter with an EPS output, or an AC-coupled battery system with its own backup gateway, is what runs the isolated island. The solar inverter does not have to be a hybrid if the battery system provides the backup.
- A battery with charge. Backup draws from the battery, so a system sitting at a low reserve gives you little runtime. Many setups let you hold a minimum reserve back for outages.
- The right circuits wired to the backup output, with a safe island earth. The backup output feeds only what is connected to it. Island operation also needs its own earth reference and, depending on the supply and inverter, a neutral-to-earth bond made only while the inverter is islanding. On the Sunsynk ECCO the manual makes an earth rod a documented condition of running the backup output. This is electrician work, not a plug-in job.
The three ways to build real backup
Backup is not one product. The designs differ mainly in how much of the house stays on, and the first two are often powered by the same inverter backup output, just distributed to more circuits. The table sums up the scope, rough cost and catch of each; the notes below add the detail that decides which one is worth paying for. The costs are rough planning estimates, not quotes.
| Backup scope | What stays powered | What it needs beyond the inverter | Rough cost (planning estimate) | Watch out for |
|---|---|---|---|---|
| Dedicated backup socket or circuit (EPS) | One or two circuits you hard-wire | Cabling from the backup terminals and a suitable island earth | A few hundred pounds of electrician work on an existing hybrid | Everything not on that output stays dead |
| Selected circuits (backup consumer unit) | A chosen set: fridge, lights, sockets, router, boiler controls | A backup consumer unit or changeover, from the same backup output, wired by an electrician | Often several hundred to low four figures | Total draw must stay under the inverter’s backup rating |
| Whole-consumer-unit backup (gateway) | The whole house, automatically | An automatic transfer switch or gateway, for example a Tesla Backup Gateway 2 | Usually the highest cost, often a few thousand pounds on top of the battery | Big loads still exceed the inverter’s power limit |
The inverter’s own backup output
Many UK hybrid inverters already include a backup output. The cheapest way to get any backup at all is to wire that output to a single circuit or a small dedicated socket, add the island earth arrangement, and stop there. The hardware is already inside the inverter, so you are paying an electrician for the wiring and the earth, not for a new box. The trade-off is that only what you hard-wired stays on. A fridge, a router and a couple of lights is a realistic scope for this approach.
Essential-circuits backup
The middle option feeds a chosen group of circuits through a backup consumer unit or a changeover switch, usually from the same inverter backup output, so lights, sockets, the fridge, the broadband router and boiler controls all come back automatically when the grid drops. This is the design many homeowners want, because it keeps the house liveable without trying to power everything. It costs more than a single circuit because of the extra board and wiring, and the combined draw of those circuits must stay under the inverter’s backup power rating.
Whole-home gateway
The top option puts an automatic transfer switch, often called a gateway, between the grid and your whole consumer unit. When the grid fails, the gateway isolates the property and the battery runs the entire house. Tesla’s Backup Gateway 2 paired with a Powerwall 3 is a common example, detecting the outage and switching the whole home to backup automatically. It is usually the most expensive of the three designs, and it still cannot get past the inverter’s power limit: a single battery system supplies a fixed continuous power, so whole-home backup does not mean you can run an electric shower and an oven together during an outage.
How long will it actually run?
Runtime is close to simple arithmetic: usable battery energy divided by the load you are running, minus the conversion losses. The catch is that people picture their normal electricity use, not the trimmed-down island a backup system actually supports.
Take a realistic essentials load. A cycling fridge-freezer, a broadband router and a handful of LED lights draw somewhere around 0.2 to 0.3 kW on average. Divide a 5 kWh usable reserve by a 0.3 kW load and you get about 16 hours on paper. Allow for the inverter’s conversion losses and its own standby draw, and roughly 15 hours is a fairer planning figure. A 10 kWh reserve roughly doubles that. Treat these as illustrative estimates from the stated loads, not measured runtimes. During daylight the picture improves on many hybrids, because the panels can keep feeding the backup output and recharge the battery while it powers the island, so a sunny day can stretch runtime well beyond the battery figure alone. At night you are on the battery only.
High-power appliances change the sum completely. An electric shower can draw 8.5 kW and a kettle around 3 kW. A common single-phase hybrid backup output is limited to a few kilowatts: the Sunsynk 3.6 ECCO, for example, is rated at 3,600 W continuous on its backup output, so an electric shower simply overloads the backup and trips it rather than running slowly. The honest planning rule is to size backup around low, steady loads and accept that heating and cooking on resistive elements are out of reach during an outage unless you have deliberately engineered for them.
A backup output is not a UPS
There is a gap between the grid failing and the backup output taking over, and how long that gap lasts varies by product and mode. Sunsynk’s own ECCO documents do not state a transfer time at all, while GivEnergy’s EPS guide documents around five seconds in standard mode. Anything that cannot tolerate a brief break in supply will blink off and restart in that window. Desktop computers reboot, some routers drop their connection, and sensitive electronics reset. Lights, a fridge and phone chargers will usually tolerate a short interruption. A PC mid-save will not.
If you need genuinely uninterrupted power for a specific device, put a small uninterruptible power supply (UPS) on that device and let the solar backup give it hours behind the UPS’s milliseconds. The two layers do different jobs: the UPS covers the switchover instant, the battery covers the duration. Treating the inverter’s backup output as if it were a UPS is the mistake that leaves people surprised when a power cut still reboots their computer.
What to ask before you buy
Backup capability is easy to assume and easy for a quote to leave vague. Settle these questions in writing before you sign, because retrofitting backup after the install costs more than specifying it up front:
- Is the inverter’s backup output being wired, or only the grid-tied side?
- Which specific circuits will stay powered in an outage, and where is that written down?
- What is the continuous power limit of the backup output, and which appliances will overload it?
- Is an earth rod or the required neutral-earth arrangement included for safe island operation?
- Will a minimum battery reserve be held back for power cuts, or will normal use empty it first?
- Does the changeover happen automatically, or do I have to switch something over by hand?
If a salesperson cannot answer the continuous-power and which-circuits questions plainly, treat the backup claim as marketing rather than a specification. A real backup design names the circuits and the power ceiling.
Common questions
Will a power cut backup keep my gas boiler running?
Yes, if the boiler is wired to the backup output. A gas combi’s electronics and pump draw only tens of watts, so a backed-up socket or circuit keeps central heating and hot water working through an outage. For most homes that is the single most valuable circuit to back up in winter, and it costs almost nothing in battery runtime. Make sure the installer puts the boiler’s circuit on the backed-up side.
Will my battery still charge from solar during a power cut?
On many hybrids, yes. Once the inverter is islanding, the panels can power the backed-up circuits and top up the battery during daylight, which extends how long you last. The exact behaviour depends on the inverter and how much solar is available, so treat a sunny outage as a bonus rather than a guarantee, and size the battery for the load you need after dark.
Can I add backup to a system I already have?
Sometimes. If you already have a hybrid inverter with an unused backup output, an electrician can often wire a backup circuit or an essential-circuits board and add the earth arrangement. If your system uses a plain grid-tied string inverter with no battery, adding backup usually means new hardware, so it is closer to a fresh install than a small upgrade.
Do plug-in or microinverter solar systems work in a power cut?
A normal grid-tied microinverter or plug-in-solar system without any backup equipment shuts down the instant the grid fails, just like any other grid-tied inverter, because it carries the same loss-of-mains protection and has no isolated island. Some microinverter systems can provide backup, but only when they are installed with their manufacturer’s own controller and battery hardware. On its own, a plug-in panel or a microinverter array cannot power your home during an outage.
Sources
- Energy Networks Association, Distributed Generation Connection Guide for G98 and G99 (March 2025): grid-tied generation must include loss-of-mains protection and disconnect from the network when the mains supply is lost. Great Britain uses G98 and G99 Issue 2 (2025); Northern Ireland uses the corresponding G98/NI and G99/NI.
- Sunsynk, ECCO SYNK-3.6K-SG04LP1 user manual: backup (EPS) output, island-mode neutral-earth bond, earth-rod requirement, and the 3,600 W continuous backup rating.
- Tesla, Powerwall 3 datasheet (UK): whole-home backup via Backup Gateway 2 and the continuous power rating.
My first-hand basis is owning and running the Sunsynk 3.6 ECCO and Fogstar battery. I have not wired or commissioned the inverter’s backup output, and I have not recorded my own house riding through a full outage, so the earthing and wiring points come from the manual and the runtime figures are arithmetic from the stated loads, not a measured blackout test.
