It sounds obvious: you have a battery full of electricity, the street goes dark, your house stays lit. Yet for many installed systems exactly the opposite happens, and finding out during your first power cut is no fun. Backup is a feature you have to specify, wire and pay for. Here's how it really works.
Why batteries switch off when the grid fails
Every grid-connected inverter in the UK is required to shut down when it detects a grid outage. The rule is called anti-islanding, and it exists to protect the engineers repairing the fault: a street that's supposed to be dead must not be quietly energised by somebody's solar system. Your battery isn't broken when it does this. It's complying with the law.
Backup means giving the system a way to disconnect itself from the grid first, then run your home as a little island behind that disconnection. That requires extra hardware (a changeover or gateway device, and sometimes rewiring of circuits), which is why it never comes free by default.
The three levels of backup
Level one is none at all. The battery works for bill savings only and shuts down with the grid. Plenty of systems are sold this way, and if cuts are rare where you live that can be a perfectly rational choice.
Level two is an EPS, or emergency power supply: a dedicated socket or a small set of essential circuits that stay live in an outage, typically covering the fridge, broadband, some lights and phone charging. Most decent hybrid systems offer this for a modest extra cost. Switchover may take a few seconds. Level three is whole-home backup, where a gateway isolates the entire house and the battery carries everything, switching over so fast you may only notice the microwave clock. This is the premium option and needs both a capable system and a meaningful chunk of installation work.
What the popular systems actually offer
The Tesla Powerwall 3 is the best-known whole-home option in our catalogue: paired with its Backup Gateway it will run the entire house, and with 11kW of continuous output it can start ovens and heat pumps rather than just keeping the WiFi alive. Sigenergy's SigenStor offers strong backup with very fast switchover in its higher configurations. FoxESS systems provide EPS-style backup on selected circuits, as do most Growatt and SolarEdge setups, with the details depending on the inverter pairing.
The specification to read is continuous backup power in kW, not just stored energy in kWh. A battery holding 10 kWh but able to discharge at 3kW will keep essentials going for hours, but it cannot start a 7kW shower no matter how full it is.
How long the stored energy lasts
In backup mode your consumption drops to whatever you choose to run. A fridge, router, lights and a TV draw perhaps 300 to 500 watts together, so a 10 kWh battery covers an evening-long cut many times over. Add an electric oven or immersion heater and the same battery can empty in under three hours.
If outage resilience is the main reason you're buying, also ask whether the system can recharge from solar while islanded. Some can run indefinitely through a long outage by topping up from the panels each day; others cannot charge at all until the grid returns.
Is backup worth paying for?
Be honest about your outage history. Most urban UK homes lose power for well under an hour a year, and paying four figures for whole-home backup to cover one brief flicker is sentiment, not economics. The maths changes in rural areas on long overhead lines, for homes with medical equipment, freezers full of expensive food, or anyone working from home on unreliable supply.
A sensible middle path for most buyers is choosing a system where EPS or gateway hardware can be added later, then living through a winter before deciding. You'll know soon enough whether your patch of the network deserves the investment.