It’s the most common surprise for new solar owners in San Diego: the grid goes down on a sunny afternoon, and the house goes dark anyway. Here is why that happens, and what has to be added for solar to keep your home running.
Key takeaways
- A grid-tied solar system without a battery shuts off during an outage, even on a sunny day. It’s designed and required to, to protect line crews.
- Under IEEE 1547, a grid-tied system has to stop energizing the grid within two seconds of an outage. This is called anti-islanding.
- A battery with a backup switch isolates your home from the grid, so the solar can keep running and recharging the battery.
- A Powerwall 3 stores 13.5 kWh, delivers up to 11.5 kW continuously and accepts up to 20 kW of panels through its built-in solar inverter.
- A battery can usually be added to existing solar, and on normal days it covers SDG&E’s expensive 4 to 9 p.m. hours.
Frequently asked questions
Do solar panels work during a power outage?
Not on their own. A standard grid-tied system shuts down when the grid does. A system with a battery and a backup switch keeps powering the home, and it can keep doing it for days as long as the sun recharges the battery.
Why does my solar shut off when the power goes out?
It’s a safety requirement called anti-islanding. Utility crews expect de-energized lines, so the IEEE 1547 standard requires a grid-tied system to stop energizing the grid within two seconds of an outage, and inverters are tested and certified to that behavior under UL 1741.
Can I add a battery to my existing solar system?
Usually, yes. The design depends on your current inverter and electrical panel, so that’s the first thing an installer should look at.
Will solar recharge my battery during an outage?
Yes, with a battery like the Powerwall 3, which has a built-in solar inverter. Once the backup switch isolates your home, the panels power the house and refill the battery during the day. Tesla says that if the battery runs down below about 10 percent, it goes into standby and tries to restart from solar each hour during the day.
Should I choose whole-home or partial backup?
Whole-home backup keeps every circuit on the battery and is the most seamless. Partial backup moves essential circuits to a backup panel, so the battery lasts longer. The right choice depends on how long outages run in your area and what you can’t do without.
The panels are on the roof. The sun is out. SDG&E has a Public Safety Power Shutoff in effect, or a transformer blew down the street. And your home, with its solar system producing at full output, has no power.
Nothing is broken. That’s how a standard grid-tied solar system is designed to behave, and it’s required to.
The short answer
A typical solar system without a battery does not power your home during an outage. A solar system with a battery and a backup switch does, and it can keep doing it for days as long as the sun keeps recharging the battery. That pairing is what people mean by solar battery backup, or blackout protection, in San Diego.
Why grid-tied solar shuts off
Most San Diego solar systems are grid-tied: the panels feed an inverter, the inverter feeds your home, and anything extra flows out to SDG&E’s lines. When the grid goes down, that last part becomes a hazard. Utility crews working to restore power expect de-energized lines. A solar system pushing electricity back onto those lines could injure them.
So the industry standards that govern inverters require them to detect an outage and stop exporting quickly. Under IEEE 1547, a grid-tied system has to cease energizing the grid within two seconds of an outage. Inverters are tested and certified to that behavior under UL 1741. This protection is called anti-islanding, and it’s the reason your panels go quiet the moment the grid does, no matter how sunny it is.
What a battery adds
A battery fixes the problem by changing what the system is connected to. With a Tesla Powerwall 3, a Backup Switch installed at the meter disconnects your home from SDG&E’s lines automatically when the grid fails. Once your home is physically isolated, there’s no risk to line crews, and your solar system is allowed to keep running inside that small, self-contained “island.”
From there, the pieces work together:
- The battery runs the house. Each Powerwall 3 stores 13.5 kWh and delivers up to 11.5 kW continuously.
- The panels recharge the battery. The Powerwall 3 has a built-in solar inverter that accepts up to 20 kW of panels, according to Tesla’s datasheet, so daytime production goes into the battery and your home.
- The system manages itself. Tesla says that when the battery is nearly full during an outage, the Powerwall tells the solar inverter to throttle back or shut off, since there’s nowhere for extra power to go. If the battery runs down below about 10 percent, it goes into standby and tries to restart from solar each hour during the day.
The U.S. Department of Energy puts it simply: for solar to keep running in an outage, it needs a properly configured inverter and a storage system. Its solar and resilience basics page explains the concept.
Whole-home backup or partial backup
When you add a battery, you also decide what it powers:
- Whole-home backup keeps every circuit on the battery. It’s the most seamless option, and Tesla says a Powerwall 3 system can provide whole-home backup to most homes. Large loads like central air, pool pumps and EV charging will drain the battery faster.
- Partial backup moves essential circuits (refrigerator, lights, internet, medical devices, a few outlets) to a backup panel. The battery lasts longer because it isn’t feeding everything.
Neither is better in general. The right answer depends on how long outages run in your area and what you can’t do without.
What about the solar you already have?
If you have solar and no battery, you’re not starting over. A battery can usually be added to an existing system, but the design depends on your current inverter and electrical panel, so that’s the first thing an installer should look at. Under SDG&E’s current solar billing rules, a battery also earns its keep on normal days by covering the expensive 4 to 9 p.m. hours, so the added backup comes with bill savings attached.
What a multi-day shutoff looks like with solar and a battery
Here is how a well-designed system behaves through a two-day Public Safety Power Shutoff:
- Before: Tesla’s Storm Watch sees the red flag warning and charges the battery to full.
- Shutoff begins: the Backup Switch disconnects the home and the battery takes over automatically.
- First evening and night: the battery runs the house.
- Next day: the panels power the home and refill the battery.
- Restoration: when SDG&E re-energizes the line, the system reconnects to the grid automatically.
SDG&E says shutoffs can last from several hours to multiple days, which is why the daytime recharge matters as much as the battery itself.
Make your solar work when you need it most
If your panels go dark when the grid does, a battery is the missing piece. Stellar Solar can look at your existing system, or design a new one, so your home keeps running through the next outage. Stellar Solar has been San Diego’s trusted solar installer since 1998, A+ rated with the BBB and a repeat winner of San Diego’s Best Solar in the Union-Tribune Readers Poll. Call 866.787.6527 for a free evaluation, or get your free quote here.
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