Solar battery backup in a power cut: why most systems switch off

Quick answer: Most solar and battery systems in the UK shut down the moment the grid fails. Connection rules (G98/G99 anti-islanding) force inverters to disconnect for safety. Real backup needs extra hardware: EPS wiring for selected circuits typically adds £300 to £600, and whole-home gateway setups can add £2,000 or more.

Why does your solar switch off in a power cut?

It surprises almost everyone. You’ve got panels on the roof and a charged battery on the wall, the street goes dark, and so does your house. A standard grid-tied solar and battery system gives you nothing in a power cut.

The reason is anti-islanding. Under the UK’s G98 and G99 connection rules, a grid-tied inverter must shut down as soon as the grid drops, as Green Hat Renewables explains. If your inverter kept pushing power into the network, the “dead” lines an engineer is repairing would still be live. So the whole system stands down until the grid returns.

What is EPS or backup mode?

EPS stands for emergency power supply. A battery system with EPS detects the outage, disconnects itself from the grid, and forms its own small island to power some or all of your house. That islanding hardware is exactly what makes it legal.

Speed matters. Tesla and Fox ESS systems switch over within about 20 milliseconds, fast enough that your router and TV never notice, while Sigenergy claims a 0ms transfer on the SigenStor. The wiring to put chosen circuits on backup typically costs £300 to £600 on top of the battery install, per Green Hat Renewables.

Single-circuit or whole-home backup?

Single-circuit (or socket) backup keeps the essentials alive: fridge, router, some lights, phone charging. It’s the cheap option and, honestly, it covers what most households actually miss during an outage.

Whole-home backup uses a gateway that isolates your house at the supply point and runs everything from the battery. It’s the premium option, and it only works well when the battery’s output can carry your real loads. An 11kW-output Powerwall 3 can island a whole house; a 3.6kW budget hybrid can’t run a kettle and much else at once. High-draw appliances are the enemy either way: a 9.5kW shower would flatten a full 13.5kWh battery in under 90 minutes.

Which systems offer real backup?

Sources: Green Hat Renewables, Heatable, Fox ESS UK, Solar4Good. GivEnergy’s All in One with Gateway 2 also offers whole-home backup, but the company entered administration in April 2026, so we’ve left it out of buying recommendations.

Backup isn’t free even on capable kit. Solar4Good’s Sigenergy pricing lists an 18kWh system at £6,000 to £7,500 installed. With backup protection it rises to £9,600. Budget roughly £2,000 to £3,500 extra for a proper whole-home backup configuration.

How long will a battery keep the lights on?

Longer than you’d think, if you’re sensible. Green Hat’s worked example runs a 9.5kWh battery at a 500W essentials load for about 19 hours, but that assumes you drain it flat. Hold back a practical 20% reserve and you have about 7.6kWh to use. Running essentials at around 500W, that’s roughly 15 hours of cover. A 13.5kWh unit on the same basis clears 21 hours.

Boil a kettle and you’re using six times that load. In a real outage you triage: fridge and router yes, oven and shower no.

Is backup worth paying for?

For most urban homes, no. UK outages are rare and usually short, and £2,000+ buys a lot of candles, or more usefully, more battery capacity. We’d put the money into kWh and accept the odd dark evening; our guide on whether battery storage is worth it runs the wider numbers.

Backup earns its cost in three cases: rural homes on overhead lines, anyone running medical equipment, and full-time home workers who can’t lose a day. If that’s you, choose a system with backup designed in rather than bolted on, and put four questions to every installer before you sign.

Which circuits stay live in an outage? What’s the switchover time? Can the panels recharge the battery while the grid is down? And what does the backup hardware add to the quote as a separate line? We compare the field in our home battery guide, and our pieces on solar panels in power cuts and preparing for power cuts cover the practical side.

FAQs

Will my battery recharge from solar during an outage?

Only on systems that form a proper island. A gateway-based setup like the Powerwall’s lets the inverter keep operating off-grid, so panels can keep charging the battery. Enphase’s Sunlight Backup does similar. Basic EPS setups vary, so ask the installer this exact question.

Can I add backup to my existing battery?

Sometimes. If your inverter has EPS terminals, an electrician can wire a backed-up circuit for roughly £300 to £600. If it doesn’t, backup means changing hardware, and at that point it’s rarely worth it on its own.

Does backup come as standard on new systems?

No, and this is the question to ask before you sign. Fox includes EPS on the EVO; Tesla needs the Backup Gateway; SolarEdge needs an add-on module. Same battery brochure, very different power-cut behaviour.