A standard grid-connected solar system shuts down during a blackout, even in full sun, because its inverter is designed and required to stop supplying power when the grid disappears. To keep the lights on, a system needs three extra things: an energy store such as a battery, an inverter that can create its own voltage rather than follow the grid's, and a switching arrangement that physically separates the home from the network before backup power starts. Together these allow the home to operate as a safe, isolated island.

Why grid-tied solar must switch off

The main reason is safety. During an outage, network crews may work on lines they expect to be dead. If rooftop inverters kept pushing power into the local network, that energy could flow back through transformers and energise lines at dangerous voltages. Anti-islanding protection prevents this, and Australian Standards for grid-connected inverters require it.

There is also a technical reason. A conventional solar inverter is grid-following: it measures the grid's voltage waveform and injects current in step with it. With no grid there is nothing to follow. Solar output also rises and falls with passing clouds, and without a battery there is nothing to balance it against the home's changing load, so voltage and frequency could not be held stable.

How anti-islanding detects a blackout

Inverters use several detection methods together:

• Voltage and frequency limits. If either moves outside a permitted range, the inverter disconnects.
• Rate of change of frequency. A sudden frequency shift suggests the stabilising influence of the wider grid has gone.
• Active methods. The inverter deliberately nudges its own output. Connected to a strong grid, the nudge has no visible effect; on an isolated circuit it causes a measurable drift that triggers disconnection.

Disconnection must happen within a short, standard-defined time, typically no more than a couple of seconds. When the grid returns, the inverter waits until voltage and frequency have been stable for a set period, commonly a minute or more, before reconnecting.

What a backup-capable system does differently

A backup-capable hybrid inverter, described in how a hybrid inverter works, keeps its anti-islanding protection towards the grid. The difference is what happens next. The changeover sequence typically runs like this:

1. The grid fails and the inverter detects the loss.
2. A contactor or transfer switch opens, isolating the backup circuits from the network. Interlocking ensures the home cannot be connected to the grid and the backup supply at the same time.
3. The inverter switches to grid-forming mode, generating its own stable voltage and frequency from the battery.
4. Backup circuits are re-energised, either almost instantly or after a short pause, depending on the design.
5. While islanded, available solar supplies the loads and recharges the battery.
6. When the grid returns and stays stable, the inverter synchronises with it, closes the grid connection and resumes normal operation.

The length of the pause in step 4 varies between designs. Some systems transfer in a few hundredths of a second, fast enough that most computers, modems and clocks keep running. Others take a few seconds, long enough for devices to restart and digital clocks to reset. If uninterrupted operation matters, for example for a home office or network equipment, check the transfer time stated in the specification rather than assuming.

Earthing and residual current protection must keep working when the supply comes from the inverter instead of the network. Getting those arrangements right is one reason backup systems must be designed by an accredited installer and wired by a licensed electrician.

Solar recharging during an outage

How solar keeps contributing in island mode depends on the architecture. In a DC-coupled system the panels feed the same hybrid inverter that is forming the island, so it simply manages the flow into the battery. In an AC-coupled system a separate solar inverter sits on the backup circuit. The battery inverter forms the island, and when the battery is nearly full it raises the island frequency slightly, which signals the solar inverter to reduce its output so the battery is not overcharged. The two designs are compared in AC-coupled vs DC-coupled batteries. Where an older solar inverter cannot respond to that signal, it may need to stay offline during a blackout.

Backup options and their limits

Three numbers set what backup can realistically do. The inverter's continuous backup rating, in kW, limits how much can run at once, and its short-term surge rating decides whether motors such as pumps and compressors can start. The battery's usable energy, in kWh, limits how long loads can run. The reserve setting, the share of the battery held back for outages, determines how much energy is available when a blackout arrives in the evening. Three-phase homes should also check whether backup covers every phase or only one. A practical starting point is to list the loads you would want during a long evening outage, add up their running power and the energy they use over several hours, and compare those totals with the inverter and battery ratings.

A portable generator must only ever connect through an approved inlet and changeover switch installed by a licensed electrician, never by back-feeding a power point. How far any of these options takes a household towards independence from the network is discussed in grid independence: what it really means.

Next steps

Backup can be planned in stages. The Blue Energy Solar market lists an Essential-Load Backup Circuit from $990 when added to a battery installation, and a Blackout Protection Package combining a battery, backup-capable inverter, essential circuits and automatic changeover from $10,490 after the federal battery discount. Prices are indicative and confirmed after a site assessment. To work out which loads matter in an outage and what system suits them, request a free assessment from Blue Energy Solar.

Frequently asked questions

Will backup switch on during a brownout or voltage sag?

Usually, if the disturbance lasts long enough. The inverter watches voltage and frequency continuously, so a sustained sag or unstable supply outside the permitted range triggers the same disconnection as a full blackout, and a backup-capable system then islands. Very brief dips may pass without a changeover. Lights flickering while the system responds is normal, and the sequence reverses once the grid is stable again.

How should I prepare a battery for a planned network outage?

If your distributor notifies you of planned work, raise the backup reserve in your monitoring app a day or so beforehand so the battery starts the outage close to full. Avoid running heavy discretionary loads during the outage, and remember that solar may recharge the battery while islanded if the design allows it. Return the reserve to its usual setting afterwards to keep everyday savings.

Should a backup system be tested?

Yes. Ask your installer to demonstrate a changeover at handover so you know what to expect, including which circuits stay live and how long any pause lasts. Many systems can also simulate an outage by following a documented shutdown procedure. Periodic checks during servicing confirm that the reserve setting, changeover device and backup circuits still work as intended, without anyone opening the switchboard themselves.