The battery as insurance: what we learned on 28 April

On 28 April 2025, at 12:33 midday, the Iberian Peninsula experienced something the textbooks considered practically impossible: the electrical zero. Within seconds, the whole of Spain and Portugal lost supply. Trains stopped, telephony down, traffic lights dark, shops closing mid-service. Restoration took all day and part of the night; full normality didn't return until the next day.

The subsequent investigations —the Government committee, the CNMC and the European ENTSO-E expert panel, which published its final report last spring— agreed on the diagnosis: an overvoltage of multifactorial origin, with insufficient voltage control that day, prior oscillations and generation trips that should never have happened, all chained into an escalation that overwhelmed the system. No cyberattack, no sabotage: a technical perfect storm, and the first documented overvoltage blackout. The system is being reinforced as a result —more dynamic voltage control, more grid storage—, and an electrical zero is unlikely to repeat for a long time. But that day left a lesson to thousands of households that this article wants to explain properly, because it's the one we've been asked most often ever since.

The bitter discovery: having panels doesn't mean having power

At 12:33 that Monday the sun was glorious. And thousands of homes with rooftop solar were left just as dark as the ones next door. The surprise —and the frustration— was enormous: "how can it be that with the sun up and brand-new panels I don't have a single light bulb?".

The answer is textbook, but nobody explains it when they sell the installation: a grid-connected solar inverter is legally required to disconnect when the grid goes down. This is anti-islanding protection, and it exists for an excellent reason: to stop your installation from feeding voltage into a line where crews may be working to restore service. Without it, every solar roof would be a hazard to the repair teams. So when the grid fell, every grid inverter in the country did exactly what it was supposed to: shut off.

The only homes that kept their lights, fridge and router that day were those with a battery and a hybrid inverter able to work in island mode: physically disconnecting from the grid and creating their own domestic microgrid, fed by the battery and, in well-designed systems, by the panels too. The difference wasn't having solar; it was having storage with real backup.

What "real backup" means

Here comes the part no brochure explains, and it's exactly where a thought-through installation is set apart from a sold one. Because "the battery has backup" can mean very different things.

The first requirement is a hybrid inverter with a backup output (EPS or backup, depending on the maker): a separate physical output that activates when the grid drops. Switchover times range from milliseconds —imperceptible transition, computers don't even notice— to a few seconds depending on the model; for a home, both are fine, but it's worth knowing which you have.

The second is the critical-circuits panel: deciding, on installation day, what hangs off the backup output. The island output has limited power —typically between 3 and 6 kW depending on the equipment—, and trying to power the whole house with everything on is the recipe for the system to trip on overload in the first minute. Sensible design separates the essentials: fridge and freezer, lighting, router and comms, kitchen sockets, the boiler or heating circulators, the garage door. The rest can wait.

The third, and the most overlooked, is that the system can recharge the battery from the panels during the blackout. It sounds obvious and it isn't: some equipment's island mode runs only on what's in the battery, and when that's gone, it's gone. A well-designed system keeps solar production active in island mode, regulating it against consumption and the battery, and can last indefinitely while the sun shines. The inverter also needs to do a black start (powering up with the grid down and the battery as the only source), and the installation must correctly resolve the neutral-and-earth regime in island mode — an invisible piece of craft that, done badly, leaves the residual-current protections blind exactly when they're most on their own.

And the fourth isn't technical but a matter of habit: test it. A backup that's never been tested is a hope, not insurance. We run a controlled disconnection test at commissioning and recommend repeating it once a year. Flipping the main breaker on any given Sunday and checking that the fridge keeps humming is thirty seconds that are worth the whole system.

How much reserve you need: the numbers

The next question is always the same: "and how long would it last me?". Let's run the numbers on a typical home's critical loads: the fridge and freezer are around 1-1.5 kWh a day; LED lighting, router, phones and TV, about 0.5-1 kWh; heating circulators or other small essentials, 0.5-1 kWh more. A home's critical loads add up to between 2 and 3.5 kWh a day — a surprisingly small fraction of normal consumption.

With a 15 kWh battery and a reserve set at 20-30% —the system always keeps that untouchable fund for emergencies, while the rest works every day on self-consumption and arbitrage—, you have between 3 and 4.5 kWh reserved: a long day of critical loads even without sun. If you're unsure what capacity suits you, we broke it down in the guide Which solar battery to choose in 2026. And let's put a price on it, using the figures behind the real cost of self-consumed energy: with grade-A LFP cells at €135/kWh, that emergency fund is worth between €400 and €600. It's probably the best-invested kWh in the whole installation. With a 30 kWh battery, the reserve covers two or three days. And if the system recharges from the panels in island mode, the question "how long does it last" changes its answer: it lasts as long as the sun shines, which where we live is nearly always. On 28 April, the well-equipped homes spent the day with an almost uncomfortable normality: the battery covered the morning, and the panels did the rest.

Insurance you'd already half paid for

On this blog we argued long ago that a battery is, above all, insurance against the volatility of the electricity price. The summer of 2026 is proving it again: with the wholesale market touching three-year highs, the regulated bill up 17% in June and 19% in July, and evenings above €0.24/kWh while solar middays brush zero, the policy's first cover works every day. On 28 April it added the second: against a supply interruption. And the best news is what that second cover costs once you've already decided to install a battery: the hybrid inverter is already in the budget, so the difference between an installation with well-resolved backup and one without is usually the critical-circuits panel and its wiring: between €300 and €600. It's probably the cheapest insurance premium you'll ever sign.

Let's be honest about the small print, as always: a blackout like 28 April is a rare event, and nobody should buy a battery out of fear alone. Island mode has limits — you won't run the whole house's air conditioning, or charge the car, on 4 kW of backup output. And a battery without backup is still a great investment for everything it does the rest of the year. But if the installation is new, not asking for the backup is leaving the best policy on the table; and if you already have a battery, the pertinent question is a different one: do you know for certain what your system would do if the grid went down right now? If the answer is "I think so...", write to us: reviewing your island mode, with a disconnection test included, is part of the free analysis. Better to find out on a quiet Sunday than on a Monday at 12:33.

SolarBox Team