Every certified plug-in solar inverter disconnects from the grid within 200 milliseconds of detecting a grid failure. This is anti-islanding protection and it is mandatory. Understanding why it exists explains several things about how plug-in solar behaves.
What islanding is
Islanding occurs when a generator continues to power a section of the distribution network after the network supply to that section has been cut. A DNO engineer who has opened a switch to de-energise a section for maintenance work expects those cables to be dead. If a solar inverter is still energising the local cables through a house connection, that section is not dead — it is islanded, still carrying voltage. An engineer working on what they believe is a dead cable and encountering unexpected live voltage is in serious danger.
Anti-islanding protection prevents this by requiring every grid-connected inverter to monitor the grid and disconnect within milliseconds when it detects the grid has failed or been switched off.
How anti-islanding detection works
The inverter continuously monitors voltage, frequency, and phase of the grid supply. Normal UK grid parameters: 230V ±10%, 50Hz ±1%. When the grid fails or is switched off, these parameters shift outside normal ranges faster than an inverter running alone can maintain them. The inverter detects the shift and trips the output relay in under 200 milliseconds — imperceptibly fast. EN 50549-1, the European standard that certified UK inverters must meet, defines the specific detection thresholds and maximum trip times.
What this means for users
Anti-islanding is why plug-in solar stops in a power cut — the inverter detects the grid has gone down and disconnects. This is correct, safe behaviour. The system reconnects automatically when grid power is restored and the inverter has verified the supply is stable (typically within 30 to 90 seconds).
It is also why the EcoFlow STREAM cannot use its grid-feed inverter output during a power cut even with a full battery. The grid-feed inverter is tied to the grid-sensing circuit. The STREAM’s direct AC outlets bypass this — they run from the battery independently of the grid-sensing inverter and work during a cut.
For everything about what happens during a power cut, see our power cut article.