Glossary · Safety and certification

What Is Backfeeding? Why Solar Must Shut Off in an Outage

BackfeedAnd anti-islanding

Power flowing backwards from a home device into a circuit or the grid. Anti-islanding is what makes an inverter stop exporting when the grid goes down.

  1. PanelsStill making power in the sun.DC
  2. InverterSenses the grid is down and stops exporting.ACConnection cut
  3. Home circuitNo solar power fed in until the grid is back.Connection cut
  4. GridDown for an outage, or for line repairs.
In an outage, a grid-connected inverter must stop sending power out, so it can't energize lines that crews are working on.

At a glance

Islanding
A generator keeps powering a spot after utility power is gone
Main danger
Shock to line workers on a line they expect to be dead
How inverters stop it
They monitor grid voltage and frequency and cease to energize
Standards
UL 1741 and IEEE 1547
In SB 868
A certified feature that prevents backfeeding the grid in an outage

Home wiring was built for power flowing one way, from the grid to your appliances. Backfeed is power flowing the other way. Into the grid during an outage, it’s dangerous: the DOE-funded guide to interconnection calls the condition islanding, a generator continuing to power a location after utility power is gone, and lists shock hazards for line workers on a line that may become unexpectedly energized as the first risk. It gives the classic example: a small gas generator illegally plugged into a wall outlet is potentially lethal to line workers.

How do solar inverters prevent it?

Grid-tied inverters monitor the utility’s voltage and frequency continuously and shut themselves off quickly when they go out of range, the guide explains; standards call it “cease to energize”. The rules come from IEEE 1547, and inverters are tested for it under UL 1741, whose test list starts with anti-islanding, according to UL.

Why is it a bigger question for plug-in solar?

A plug-in system backfeeds into a branch circuit through a receptacle by design, which UL’s white paper says creates scenarios current codes don’t address, such as combined current exceeding what the wiring is rated for. That’s why California’s SB 868 requires a certified feature that prevents the device from backfeeding the grid during an outage, and why plug-in solar alone isn’t backup power: see the California plug-in solar law.

Published by the Plugged Sun editorial team

Sources

  1. Connecting to the Grid: A Guide to Distributed Generation Interconnection Issues, 6th edition (Official document, Interstate Renewable Energy Council and NC Solar Center (DOE-funded), 2009)
  2. Enable a smarter, safer, reactive grid interconnection with UL 1741 SA (Source, UL, 2018)
  3. Plug-In Photovoltaic (PIPV) Systems: Safety Considerations for Residential Use (Source, UL Solutions, 2025)
  4. SB 868 (Wiener), as amended April 7, 2026: bill analysis (Official document, California Assembly Committee on Utilities and Energy, 2026)

Where Backfeed comes up