Islanding

The safety concern behind the term

When utility workers respond to a grid outage - to repair a fallen power line, for instance - they follow lockout/tagout procedures to ensure the section they’re working on is de-energised. If a solar inverter or battery in a nearby house is still injecting power onto that section, those wires are live even though the grid protection equipment has opened. This is the hazard that anti-islanding standards exist to prevent.

Australian standard AS/NZS 4777.1 requires that grid-connected inverters detect loss of grid and disconnect within 2 seconds. The detection methods are specified (frequency shift, voltage monitoring) and tested as part of product certification. An inverter that passes the standard will not unintentionally island.

Intentional islanding - the good kind

The term gets repurposed in a positive context when describing batteries with backup capability. “Intentional islanding” or “controlled islanding” describes a battery system that:

1. Detects grid loss
2. Deliberately disconnects the house from the grid (via an automatic transfer switch or gateway)
3. Forms its own local AC supply using the battery and inverter
4. Operates as a self-contained microgrid until the grid is restored

The key word is “controlled.” The house is physically isolated from the grid before the battery starts supplying it - so there’s no path for household-generated power to reach utility workers on the street. This is why the disconnection hardware (Powerwall Gateway, Enphase System Controller) is a required part of any whole-home backup installation, not an optional add-on.

The technical challenge

For a single battery-inverter to island successfully, it needs to be a grid-forming inverter - capable of establishing stable voltage and frequency without an external reference. Early residential batteries used grid-following inverters that simply couldn’t operate without the grid present. The shift to grid-forming architectures in products like the Enphase IQ8 and redesigned hybrid inverters has made reliable intentional islanding a standard residential capability rather than a premium feature.

During islanded operation, the inverter manages the local grid: it adjusts output to match load, handles motor starts and inrush currents, and maintains frequency stability without the inertia of large synchronous generators. This is more complex than grid-following operation, which is why intentional islanding has historically required more sophisticated (and expensive) equipment.