Will a solar battery work in a blackout?

The short answer

Plenty of people are surprised the first time the power goes out: the sun is shining on the panels, and the house is still dark.

That is normal. Solar panels on their own switch off in a blackout. A home battery can keep some or all of the house running, but only if the system was designed for backup from the start. Owning a battery does not automatically mean you have blackout protection.

Why solar switches off when the grid goes down

When the grid fails, workers may be out fixing the lines. If thousands of rooftop systems kept pushing power into those lines, the wires the crews expect to be dead could be live.

So a grid-connected solar inverter (the box that turns the panels' power into the power your home uses) is built to shut down as soon as it detects an outage. This is called anti-islanding. The rules for connecting inverters to the grid in Australia are set out in a standard called AS/NZS 4777. The federal government's energy.gov.au notes that very few grid-connected solar-only systems give any backup power at all for this reason.

A battery only helps if it is set up for backup

A battery can give you power in an outage, but it has to be configured to do it, and not every battery system can. What decides it is not only the battery. It is the inverter and how the house is wired.

A backup-capable system needs an inverter that can separate the home from the grid and run it as its own little "island". These are called hybrid or multiple mode inverters. When the grid drops, the system disconnects from the street and keeps supplying the circuits it is wired to.

Essential circuits or the whole home

Backup comes in two main flavours.

• Essential circuits. The installer moves a handful of circuits onto a separate backup board, fed from the inverter's backup output. Typically that is the fridge, some lights, and a power point for the wifi router and phone chargers. Everything else goes dark.
• Whole home. Every circuit stays on. That sounds better, but it drains the battery faster, and it does not lift the limit on how much can run at once.

If your home has three-phase power, some systems back up only one of the three phases. Ask which circuits end up on that phase.

How fast does it switch over?

The gap between the grid dropping and the battery taking over is the switchover time. It varies by product.

Some inverters switch so quickly that most appliances do not notice, a bit like the uninterruptible power supply (UPS) you might have on a work computer. As one example, Fox ESS lists a switchover time of under 20 milliseconds on its H3 Smart hybrid inverter. Other set-ups leave a short gap, long enough for clocks to reset and a computer or wifi router to restart.

If you work from home or run medical equipment, ask for the switchover time in writing. Do not rely on any home battery as the only supply for life-support equipment without advice from your equipment supplier.

What can you actually run?

A battery has two limits that matter in a blackout:

• Energy (kWh): how much it stores. This decides how long it lasts.
• Power (kW): how much it can deliver at once. This decides how many things can run together.

The power limit is the one people forget. Fox ESS lists backup output from 5 kW to 15 kW across its H3 Smart three-phase range, depending on the model, shared across the three phases. energy.gov.au advises managing your usage during an outage so you do not overload the battery inverter.

Here is roughly what common appliances draw, using figures from Queensland's Electrical Safety Office.

A fridge, lights, the wifi router, phone chargers and a TV are easy for almost any backup system. Air conditioning, electric ovens and cooktops, and pool pumps are the heavy loads. They are usually left off essential-circuit backup, and even with whole-home backup you may need to run them one at a time, or not at all, until the grid returns.

Will the solar keep charging the battery?

This is the question that decides whether you get one evening of backup or several days.

• In some systems, the solar shuts down in a blackout and only the battery runs. When the battery is flat, you wait for the grid.
• In an islandable system, the solar keeps working during the outage and recharges the battery when the sun is out. energy.gov.au describes this as the more expensive option, but one that may keep you going over multiple days if you manage your usage carefully.

If you are adding a battery to existing solar, check this carefully. Depending on how the new battery connects, your existing solar inverter may still switch off in a blackout, leaving the battery on its own.

The honest downsides

Backup is not free, and it is not right for everyone.

• It adds cost. Some batteries include backup, others charge extra for it. Moving circuits onto a backup board is extra electrical work, and Energy Safe Victoria's guidance, based on the national Wiring Rules, is that circuits moved onto a backup supply need suitable safety switch (RCD) protection. An older switchboard can turn a simple job into a bigger one.
• A backup reserve is a battery that is not earning. To be ready for a blackout, you usually tell the battery to keep some charge in reserve. As an illustration, if you hold back 30%, that 30% is not cutting your evening bill or selling into price spikes on a wholesale plan. Most nights there is no blackout, so that reserve sits idle. If you use Sunnn+, trading stays above the reserve you set, so the trade-off is yours to choose.
• Storm damage changes everything. The Queensland Government warns that if your home is impacted or damaged by storms, it is no longer safe to use your solar or battery system. Get it checked by a licensed electrician first.

When backup is worth it, and when it is not

Backup makes the most sense if you:

• live somewhere with regular outages, like the end of a long rural line or a storm or bushfire area
• work from home and cannot afford to drop off calls
• rely on a fridge full of medicine, a freezer full of food, or a water pump

If blackouts in your street are rare and short, the extra cost and a standing reserve may be worth less than putting the whole battery to work on your bill. See is a home battery worth it and how big a battery you need.

What to ask for at quote time

Backup is easier to get right before installation than after. Get these answers on the quote:

1. Does this system provide backup in a blackout, yes or no?
2. Is it essential circuits or whole home? Which circuits exactly?
3. What is the backup power rating in kW, and on how many phases?
4. What is the switchover time?
5. Does the solar keep charging the battery during an outage?
6. What does backup add to the price, including any switchboard work?
7. What backup reserve do you recommend, and how does that affect savings?

Next step

To see what a battery could do for your bills, try the savings calculator. If blackout protection matters to you, check your eligibility and tell us at the start. We design and quote the system around it, and installation is arranged through accredited installers. Savings figures are modelled estimates, not guarantees.