Solar Battery Backup for Power Cuts: Does It Keep Your Lights On?
Most people assume a home battery powers the house in a blackout. It doesn’t — not unless you ask for it. Here’s the honest guide to getting backup that works.
The short answer
Here’s what surprises most people: a standard solar battery does NOT power your home during a power cut. By default it automatically switches off — a legal safety requirement (UK G99 rules) that stops your system feeding the grid and endangering engineers working on the lines.
To keep your lights on in an outage, you need a battery with EPS (Emergency Power Supply) or backup capability, deliberately specified and installed. There are three levels: a single backup socket, selected essential circuits, or whole-home backup (via a gateway) — rising in capability and cost. Not every battery supports it, so you must choose a backup-capable battery and tell your installer up front. Get it right and your home rides through power cuts; with solar recharging the battery by day, backup can last almost indefinitely. Here’s the full guide.
What this guide covers
One of the biggest misconceptions in home solar is that having a battery means you’re covered in a blackout. Sadly, that’s usually not true out of the box. But the good news is that proper backup is very achievable — if you plan for it. Here’s exactly how it works.
Why a standard battery switches off in a power cut
It feels counterintuitive: you’ve got a battery full of energy, the grid goes down, and… your battery shuts off too. Why? Safety, and it’s the law. Under UK G99 grid regulations, all grid-connected solar and battery systems must automatically disconnect during a power cut. If they didn’t, your system could keep feeding electricity into the local network — and seriously injure or kill the engineers working to restore power, who expect those lines to be dead. So the automatic shutdown isn’t a fault or a limitation of your battery; it’s an essential safety feature. The way around it — safely — is a battery with backup capability that can isolate your home from the grid and then power it independently, which is exactly what EPS does.
What is EPS (Emergency Power Supply)?
EPS is the feature that makes backup possible
EPS (Emergency Power Supply) is the function that lets a battery safely disconnect from the grid during an outage and keep powering your home from stored energy. An EPS-capable system detects the grid failure, isolates your property from the network (so it’s safe for line workers), and switches your chosen circuits over to battery power — usually after a brief interruption of a few seconds. Without EPS (or a whole-home backup gateway), a battery simply shuts down in a cut and does nothing for you. The critical point: not every battery on the UK market supports EPS, and where they do, the level of backup varies. So if staying powered in a power cut matters to you, EPS isn’t an afterthought — it’s a feature you must choose deliberately and tell your installer about, so they size the battery, inverter and cabling correctly.
The three levels of battery backup
Backup isn’t all-or-nothing — there are three levels, rising in capability and cost:
| Level | What it powers | Relative cost |
|---|---|---|
| 1. Single EPS socket | One dedicated socket — fridge, phone, laptop | Lowest |
| 2. Essential circuits | Selected circuits via sub-board — lights, sockets, fridge | Medium |
| 3. Whole-home backup | Entire home via backup gateway — runs as normal | Highest |
A single EPS socket is the simplest and cheapest — one socket stays live so you can plug in the essentials. Essential (critical) circuits use a sub-board to keep chosen circuits running — typically lighting, key sockets and the fridge — so much of the home functions. Whole-home backup uses a backup gateway (such as Tesla’s Gateway or a Fox ESS backup box) to transfer your entire consumer unit to battery power, so the whole house runs as if nothing happened. Each level adds electrical work and cost, so the smart move is to choose the level that matches what you genuinely need to keep running — most people find essential circuits the sweet spot.
Which solar batteries offer backup?
Several leading batteries support backup — but capability varies, so this is where choosing the right model matters:
| Battery | Backup capability |
|---|---|
| Tesla Powerwall 3 | Whole-home backup as standard (with Gateway) — UK benchmark |
| Fox ESS | EPS as standard; whole-home backup box available |
| Sunsynk | Strong EPS & off-grid heritage — good for rural outage-prone areas |
| SolarEdge | Backup with a backup interface module |
| Growatt (SPH) | Backup via SPH hybrid systems |
| GivEnergy | Offers backup — but note administration in 2026 |
The Tesla Powerwall 3 remains the whole-home backup benchmark, with high output and islanding built in. Fox ESS is notable for including EPS as standard, and Sunsynk‘s off-grid heritage makes it a favourite where outages are frequent. The essential takeaway: always confirm the specific model’s backup capability and level before you buy — “has a battery” and “has working backup” are two different things, and the difference is easy to miss. If you’re weighing options, our battery sizing guide and individual reviews help.
How long will backup last in a power cut?
This depends on your battery capacity and what you’re running. As a rough guide:
| Load | 10kWh battery runtime |
|---|---|
| Essentials only (fridge, lights, phone, laptop ~0.5–1kW) | ~10–20 hours |
| Plus medium loads (TV, microwave, occasional) | Several hours |
| Heavy loads (electric shower, hob, EV charger) | Drops sharply |
A 10kWh battery running just essentials will typically keep going for 10–20 hours — comfortably through most outages. The real magic of solar-plus-battery backup: during a daytime power cut, your panels can recharge the battery as you use it, potentially extending backup indefinitely through daylight hours, as long as generation keeps pace with what you’re drawing. That’s something a battery-only or generator setup can’t match.
Is battery backup worth it?
Honestly, it depends on your situation. The average UK home sees very few outages — often well under one a year, totalling under an hour. For those homes, backup is more peace-of-mind than necessity. But it becomes genuinely worthwhile if you: live somewhere with frequent or rural power cuts; rely on medical equipment; work from home and can’t afford downtime; or simply value the security of never losing power. The extra cost of specifying EPS as standard when you’re already installing a battery is relatively modest — far cheaper than retrofitting it later. So even if you’re unsure, it’s worth discussing with your installer at quote stage, because adding it upfront costs a fraction of adding it afterwards. If you’re getting a battery anyway, at least ask the question.
Battery backup: the honest summary
✓ Good to know
- Backup IS achievable — with the right kit
- Three levels: socket → circuits → whole-home
- Solar recharges backup by day (huge plus)
- 10kWh runs essentials ~10–20 hrs
- Cheap to add upfront vs retrofitting
- EPS-capable batteries widely available
✗ Watch out for
- Standard batteries switch OFF in a cut
- Not all batteries support backup — check
- Must be specified up front, not later
- Brief few-second switchover (unless UPS)
- Whole-home backup costs the most
- Heavy loads drain it fast
Frequently asked questions
Does a solar battery work in a power cut?
Not by default — a standard grid-tied battery automatically switches off in a cut (a G99 safety law protecting line workers). It only powers your home if it has EPS/backup capability built in and set up. You must choose a backup-capable battery and specify it when ordering.
What is EPS?
Emergency Power Supply — the function that lets a battery safely isolate from the grid in an outage and power your home from storage. Without it, a battery shuts down in a cut. Not all batteries have it, so check before buying.
What are the three backup levels?
A single EPS socket (one socket stays live); essential circuits (a sub-board powers chosen circuits like lights and fridge); and whole-home backup (a gateway runs the entire home). Cost rises with each level.
Which batteries offer backup?
Fox ESS (EPS standard), Tesla Powerwall 3 (whole-home standard), Sunsynk (strong EPS), SolarEdge (with module) and Growatt SPH all support it. Always confirm the specific model’s capability and level.
How long does it last?
A 10kWh battery on essentials (fridge, lights, charging) lasts ~10–20 hours. Heavy loads drain it far faster. In a daytime cut, solar can recharge it as you use it, extending backup potentially indefinitely.
Is the switchover instant?
Usually a brief 2–3 second interruption while the system isolates and switches to battery — fine for lights and fridges. Truly seamless, zero-gap power (for medical or sensitive equipment) needs a more complex, costlier UPS setup.
Backup works — but only if you ask for it up front
The bottom line: A standard solar battery won’t power your home in a cut — it switches off by law. To keep the lights on you need an EPS/backup-capable battery, specified deliberately, at one of three levels (socket, essential circuits, or whole-home). It’s inexpensive to add when you’re installing a battery anyway, and solar recharging the battery by day makes it genuinely powerful. If backup matters to you, say so at quote stage.
Get my free backup quotes →The verdict
If there’s one thing to take away, it’s this: don’t assume a home battery keeps your home powered in a blackout — by default, it doesn’t. A standard grid-tied battery legally switches off during a power cut. But proper backup is very achievable: choose a battery with EPS or whole-home backup capability, pick the right level (a single socket, essential circuits, or the whole house), and have it designed in from the start. With solar topping the battery up during daytime outages, you can ride through power cuts almost indefinitely.
The key is to decide before you buy — adding backup upfront costs a fraction of retrofitting it, and not every battery supports it. If living without power worries you, or you’re in an outage-prone area, it’s well worth specifying. The best next step is a couple of free, no-obligation quotes from MCS-certified installers who can design a backup setup around your home and the essentials you want to keep running.
Important
Going Solar UK is an introduction service, not an installer. This guide is general 2026 information, not electrical advice. Battery backup involves specific equipment and electrical work that must be designed and installed by a qualified, MCS-certified installer in line with G99 and current safety regulations. Backup capability, levels, runtimes and costs vary by battery model, home and installer — always confirm the details for your specific system before buying.
We may receive a commission when we introduce you to an installer, at no cost to you. Any contract for work is directly between you and the installer.