Does a home battery with backup work in a power cut?

A home battery with backup power only keeps your home running during a power cut if the system is designed for it. A grid-connected home battery without a backup function actually switches off during a cut, because it may not put voltage on a dead grid. For real backup power you need an inverter that forms a local grid itself. You also need an automatic grid disconnection and a distribution board with the circuits that must keep running. There are three levels: an emergency socket, backup of critical circuits, or an off-grid capable system. Which home battery with backup suits your home depends on the level you need.

Picture a winter evening. The power goes off, the battery is full, and still the house is dark: no heating pump, no fridge, no router. The first real power cut is when you find out whether your battery has a backup function. A long cut also hits more than comfort: the freezer, the sump pump and working from home all stop.

Why does an ordinary home battery go off during a power cut?

A battery connected to the distribution grid on the AC side that delivers energy counts as a power generating unit under the technical rule C10/11 of Synergrid, which applies across Belgium. It must detect an island situation (under EN 62116) and then disconnect from the grid. The Belgian electrical safety code (AREI/RGIE) allows, for sources up to 30 kVA, an automatic disconnection system that separates galvanically within 5 seconds when the grid fails. For a household solar installation (up to 10 kVA), the accredited inspection body tests that disconnection during the inspection.

Without a backup function, battery and solar panels stop together, even with a full battery and sun on the roof. A system with real backup power first separates (part of) your installation from the grid and then powers it itself.

What is the difference between an emergency socket, backup and an off-grid capable system?

On our home battery page, you read how we connect important appliances separately to the battery. That is the difference between a real system and a storage box: during a power cut, the battery takes over from the grid.

Take a look at our home battery references, such as the home in Sint-Niklaas that runs fully on backup.

How fast does a home battery with backup power switch over?

According to its datasheet, the Victron MultiPlus-II takes over the connected loads in less than 20 milliseconds, fast enough for computers. The Huawei Backup Box-B1 states a switch-over time under 3 seconds, and with a three-phase connection it delivers only one phase in backup, with at most 3,300 VA.

A few seconds are enough to make a router restart. So ask: how many milliseconds, how many phases and how many VA in backup?

What should you look for when choosing a home battery with backup power?

• Power, not just kWh. According to its datasheet, a MultiPlus-II 48/5000 delivers 4,000 W continuously at 25 °C and 9,000 W peak. Start-up peaks of compressors count too.
• Phases. With a three-phase connection: is the backup three-phase too?
• Charging during a long power cut. For AC-coupled solar panels, Victron applies the factor 1.0 rule: the solar power may not exceed the continuous power of the inverter-charger at 25 °C. A generator input gives extra security.
• Earthing. The AREI/RGIE requires protection against electric shock to keep working when only one source feeds the installation. The MultiPlus-II does this with an earth relay that connects the neutral to the chassis as soon as there is no external supply, so the residual current device keeps working.
• Monitoring. Through the monitoring in the app and the VRM portal, you set an alert for a low battery level.

How much battery do you need for backup power through a night and a day?

Illustrative example: a home with gas heating puts its critical circuits on backup.

With 10 kWh of usable capacity, this home bridges about 1.9 days without sun. Hob, fridge and pump together need more than 2 kW plus the start-up peak of the compressor, so an inverter of 4 kW continuous has margin. A heat pump changes everything. That is why we size on measured quarter-hour consumption, never on a rule of thumb.

Which inspection and registration does a home battery with backup power need?

• Inspection (all of Belgium). Under the AREI/RGIE, adding a circuit to a household installation is a major change: a conformity inspection before use is mandatory. A backup distribution board therefore falls under it.
• Switch-over. The switch between energy sources acts on all live conductors and may not connect the sources in parallel at the wrong moment. C10/11 requires "break-before-make": disconnect first, then supply power yourself.
• Registration in Flanders. You must register a home battery with Fluvius, with a valid AREI inspection and a clear single-line diagram. After moving the battery, a new inspection is mandatory; an extra battery module does not need a new inspection according to Fluvius.
• Declaration in Wallonia. ORES requires every new installation to be declared, including a battery and plug-and-play devices. Above 10 kVA, a more thorough study is needed, and a smart meter is mandatory for any new installation. If RESA is your grid operator, check its procedure.
• Brussels. Check the declaration rules for home batteries with Sibelga.

Want to go further than backup? Then read our article on what off-grid means to see what being fully disconnected involves. Looking for backup power for a business? Other trade-offs apply there (UPS, generator, battery), which we explain in our article on backup power for businesses. Whether a blackout like Spain's could hit Belgium is in our article on blackout risk for businesses.