One homeowner's backup power question sparked a useful discussion. During a blackout, if a battery is already full but rooftop solar is still producing, how does the system deal with the extra electricity?

What's happening?

On r/SolarUK, the original poster described a setup that used a GivEnergy backup gateway alongside a Solis inverter and asked how it behaves in island mode if the grid is out and the battery has no room left to charge. Their main concern was how the solar side would respond without a signal telling it to stop.

Reddit users said the system does not dump power elsewhere. Instead, the inverter adjusts to match the demand it detects, and the gateway can alter operating conditions so the solar inverter drops offline and comes back when charging can resume.

"The inverter will just limit the output to whatever load it sees," one commenter wrote.

Another person explained, "In island mode, when the AIO battery is at 90%, it pushes the main's AC frequency slightly away from 50Hz. Not enough to cause any problems generally but enough to cause the solar inverter to detect an abnormal AC frequency and shut down — as it is required to do anyway, following G98/G99 specs.

"As soon as the battery is below 90%, it changes the AC frequency back to 50Hz, the solar inverter starts back up and charges the battery again."

Adding battery storage is one of the best ways to protect your home during outages, save money on energy, and go off-grid. If you want to compare home battery storage options and competitive installation estimates, it may be worth exploring EnergySage, which has teamed up with the electrification brand Qmerit to guarantee you get the best price on home battery storage solutions.

More background

This kind of setup can determine whether rooftop solar remains useful during a power cut.

A standard grid-tied solar system often shuts off during an outage for safety reasons, even if the sun is shining. But with the right battery and gateway hardware, homeowners can keep critical loads running while recharging.

That can make a meaningful difference in both comfort and cost. Every kilowatt-hour your home can use from its own panels during an outage is power you do not have to buy from the utility. It also helps homeowners get more value from the solar equipment they paid for.

According to one reply, systems that combine a gateway with an all-in-one battery can use this frequency-based approach to manage solar production during an outage.

A different arrangement, such as with certain AC-coupled battery inverters, may not be able to control the solar inverter that way, which means the panels could stay offline and the home would run only on stored battery power.

What can be done?

Homeowners should ask installers detailed questions before buying or expanding a backup system.

Not all batteries, gateways, and inverters behave the same way in an outage, and the difference can determine whether solar can keep charging your battery when the grid is down.

For a low-cost option, Pila offers plug-and-play batteries priced at a fraction of what a whole-home backup system costs, which could make backup power more accessible for renters or households not ready for an installation.

"Oh wow, that's really clever actually," the OP wrote.

The commenter who explained island mode said, "Quite a neat hack really."

Where can I learn more?

This question opened a conversation about what a backup system can do when the grid goes down. Batteries, off-grid planning, and extra solar production can shape how useful solar remains during an outage.

• Some solar households say full-home battery backups make outages easy to overlook.

• Off-grid homeowners say cutting ties with utilities demands constant attention to daily power use.

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