Will a Home Battery Keep the Power on in a Cut?

The question we get asked most after “how much will it save me”

Most people look into battery storage for the bill savings — storing cheap or free solar power to use at 24p/kWh peak rates instead of buying it in. But the second question, especially after a stormy winter or a local outage, is usually “if the grid goes down, will my battery keep the lights on?” The honest answer is: it depends entirely on how the system is wired, and a lot of standard installs won’t do it automatically.

Why most grid-tied batteries switch off in a power cut

This surprises people, but it’s a safety requirement, not a design flaw. When the national grid goes down, engineers need to know with certainty that no electricity is flowing back into the network while they’re working on it. Standard grid-tied inverters — the kind fitted in most solar-plus-battery installs — detect a grid outage and shut the whole system down within a fraction of a second, battery included. You lose power at exactly the moment you’d want it most, because the inverter has no way of keeping your home isolated from the grid on its own.

What actually gives you backup power

To get genuine power-cut backup, the system needs a specific piece of hardware: an automatic transfer switch, sometimes built into the battery itself, sometimes a separate gateway unit. This disconnects your home electrically from the grid the instant an outage is detected, so the inverter can safely keep generating and discharging into an isolated circuit — your home — without exporting anything back down the line. It happens automatically, usually within a second or two, often fast enough that you won’t notice more than a flicker.

Not every battery on the market includes this as standard:

• Tesla Powerwall 3 includes whole-home backup as standard — this is one of its strongest selling points, and the reason we recommend it specifically when backup is the priority rather than an afterthought.
• Sigenergy SigenStor offers backup power as an optional add-on gateway rather than a built-in default, so it needs to be specified at the point of ordering.
• FoxESS systems also support backup via a separate gateway/switch, again as an add-on rather than included by default.

If backup matters to you, say so before you sign anything — it changes the spec, and sometimes the battery recommendation, not just a setting that gets switched on later.

Whole-home backup vs partial backup

There’s also a distinction between backing up your entire consumer unit and backing up a handful of essential circuits. Whole-home backup (Powerwall 3’s approach) keeps everything running, within the limits of your battery’s power output and stored capacity. Partial or “essentials” backup routes only specified circuits — usually the fridge-freezer, some lighting, and a few sockets — through the backup switch, which stretches a smaller battery’s runtime much further during a longer outage. Which makes sense depends on your battery size and what you actually need to keep running: a household with a large battery and modest usage might comfortably run everything; a smaller battery is often better dedicated to keeping the essentials going for longer rather than running the whole house for a shorter window.

How long will it actually last?

This comes down to two numbers: your battery’s usable capacity in kWh, and how much you’re drawing per hour during the outage. A 10 kWh battery running just a fridge-freezer, router, a few lights and phone charging — call it 0.5–1 kWh per hour — could realistically stretch to 10-plus hours. Add an electric shower, kettle, or oven into the mix and that same battery might be drained in under two hours, because those appliances draw several kW each the moment they’re switched on. If backup is a priority, we size the battery around your actual outage-use case, not just your daily solar consumption pattern, and talk through realistic runtimes for your household before you commit to a size.

It’s also worth noting that a battery on backup duty isn’t recharging from solar at full strength if the outage happens on a dull day or overnight — daylight and cloud cover still apply, cut off from the grid or not. The battery is working from whatever charge it already had plus whatever your panels can add during daylight hours of the outage.

Do you need this?

Not everyone does, and it’s not a feature we’d push on every household — it’s genuinely optional hardware with its own cost. It matters most if you’re on a rural or coastal network prone to storm-related outages, if anyone in the home relies on medical equipment or refrigerated medication, if you work from home and connectivity/lighting during a cut is a real disruption, or if you simply want the peace of mind. If none of that applies, a standard grid-tied system optimised purely for savings is a perfectly reasonable choice, and a cheaper one.

Getting it right from the start

Backup power is far easier and cheaper to specify at the point of installation than to retrofit later — the transfer switch and any rewiring of backed-up circuits are part of the original job rather than a separate visit. If keeping the lights on during a power cut is part of why you’re considering a battery, it’s worth raising it early rather than assuming every system does this by default, because plenty don’t.

Our free, no-obligation survey looks at your home, your usage, and what matters most to you — whether that’s maximum savings, backup resilience, or both — and comes back with a system sized and specified for the actual outcome you want, not just a generic quote.