How Does Battery Backup Work With Solar ?

What Stays Powered in a Blackout, and What a Home Battery Needs to Deliver It

Last Updated: 2026-08-10

Key Takeaways

How does battery backup work with solar ? Only when the installer builds the system for it. A battery keeps power flowing in a blackout because backup-specific hardware disconnects your home from the grid first. Owning a battery does not do it on its own.
What happens to solar power during a blackout follows a safety rule called anti-islanding. The inverter stops supplying power the moment grid voltage disappears. No power then flows into lines that a repair crew may be working on.
Solar battery backup during a blackout needs a backup-capable inverter, or a separate automatic transfer switch. That hardware isolates the house electrically, so the battery can supply your circuits without sending power back into the grid.
Capacity and wiring usually settle the choice of whole home battery backup vs partial backup. Most residential systems back up a critical loads panel of a few essential circuits rather than the whole house.
Home battery backup in a power outage lasts as long as the usable energy in the battery divided by the draw of the backed-up circuits. That is a relationship, not a fixed number of hours.
The difference between power and energy decides what backup can run. The power rating of the inverter sets what will start. The stored energy in the battery sets how long it keeps going.
Fitting a backup battery for an existing solar system is a separate piece of work, not a setting to change. Ask whether your current or proposed system already includes the hardware for it.

What Happens to Solar Power During a Blackout ?

When the grid fails, a grid-connected solar system shuts itself off. The grid is the shared electricity supply your home connects to. The system shuts off regardless of how much sun falls on the panels, and whether or not it includes a battery.

The rule behind this shutdown is anti-islanding. It requires an inverter to stop sending electricity into the grid as soon as the grid itself loses power. Without that rule, a street of solar homes could keep local lines live while a repair crew works on them.

Why Grid-Connected Solar Switches Off

Line worker safety: a de-energised line must stay de-energised. Anti-islanding makes sure that no rooftop solar system puts power back into a line the distributor has switched off.
A standards requirement, not a setting: inverters sold for grid connection in Australia must detect the loss of grid supply and disconnect within seconds. The standard AS/NZS 4777.2 sets out this behaviour (Source: Standards Australia).
It applies with a battery too: a battery does not switch the rule off. A battery behind a standard grid-tied inverter stops supplying power along with everything else.
It says nothing about your equipment: a system that shuts down in an outage works exactly as designed. It has not failed.

Does My Solar System Work During a Power Outage ?

On its own, no. Solar panels, a standard inverter, and a battery in the garage all sit idle until the grid returns. The exception is an installation that the installer has wired and configured for backup.

If you have not lost power since the installer commissioned your system, you do not yet know whether it has backup. Ask your installer or an assessor. The presence of a battery does not answer the question.

If you would rather have a professional turn all of this into a clear, costed plan for your home, find out more about how our Energy Assessments can help you.

What a Backup-Capable System Actually Adds

Backup uses the shutdown rule in the other direction. The system disconnects your home from the grid first, then supplies power inside that disconnected boundary. Your house becomes a small, deliberate island, and engineers call that state islanding.

This needs hardware that a standard grid-tied system does not have. That hardware is an inverter built to run in backup mode, or a separate switch that isolates the house. A battery cannot do it alone, and nor can the panels.

The Three Jobs a Backup Setup Has to Do

Detect: the system senses that grid voltage has gone. It then confirms a genuine outage rather than a momentary dip.
Isolate: the system opens the connection to the grid. An automatic transfer switch does this job, or a multi-mode inverter with the same switching built in.
Supply: the battery energises the backed-up circuits. On systems that support it, the solar panels supply power as well, so daytime generation can recharge the battery during the outage.

Changeover Time — the Gap You May Notice

The handover from grid to battery is not always instant. Changeover time is the short gap between the loss of the grid and the start of the backup supply. Manufacturers publish it as a specification, because it varies between products.

Vendor documentation commonly quotes changeover times in the tens of milliseconds. That is brief enough that your lighting may only flicker. Some equipment tolerates no interruption at all, and a desktop computer, some routers, and certain appliance controllers can still restart in that gap.

Is Home Battery Backup a UPS ?

Equipment standards set the legal definition of a UPS (uninterrupted power supply), rather than marketing. Those standards generally reserve the term for a supply that carries the load with no break at all.

UPS (uninterrupted power supply): the connected load never sees the grid disappear, because the supply runs without a break through the transition.
EPS (emergency power supply): a short changeover gap, after which the battery carries the backed-up circuits. Suppliers describe most home battery backup this way.

Whole Home Battery Backup vs Partial Backup

Residential backup is commonly limited to a subset of circuits rather than the whole house. Backing up everything needs more battery capacity and different switchboard wiring than backing up a few essential circuits. That difference usually decides the design.

The installer wires the chosen circuits to their own small sub-board, called a critical loads panel. Some solar battery installers call it an essential loads sub-board instead. Everything else in the house stays dark until the grid returns.

	Partial (critical loads) backup	Whole home backup
What stays on	A chosen handful of circuits on a separate sub-board.	The house as normally wired, subject to the power limit of the inverter.
Battery capacity	Less, because the load it has to carry is smaller.	More, because every appliance in the house can draw from it.
Switchboard work	A sub-board, and a new route for the selected circuits.	A larger rearrangement, and often heavier switchgear.
Big appliances	Generally left off the backed-up side by design.	Included, which is what drives up both the battery and the inverter sizing.
Usual fit	Households that want the fridge, lights, and internet to ride through an outage.	Households that want to notice no difference, and will pay for the equipment that allows it.

Australian installer guides published through 2026 broadly describe the critical-loads approach as the common residential choice. They treat whole-home backup as the exception that needs oversized equipment.

Backing up a whole house for long periods typically requires off-grid-capable equipment. This applies particularly outside the sunniest six to nine months of the year in a temperate region such as the Central Highlands. That equipment is a different and more expensive class of system.

Essential Versus Non-Essential Loads

You decide which circuits count as essential. Make that decision deliberately, rather than accepting a default list. The split exists to keep the backed-up draw small enough for the battery to last.

Commonly essential: lighting, the fridge or freezer, the internet modem, phone charging, and a few nominated power points.
Often essential in a cool climate: the pump and controls of a hydronic or ducted heating system. They draw far less power than the heat they move suggests.
Commonly non-essential: the oven, an electric hot water element, the clothes dryer, and pool or spa equipment.
The awkward middle: reverse-cycle heating and cooling, and an induction cooktop. They are genuinely useful in a long outage, but they draw enough power to change the sizing of the equipment.
Medical equipment: if anyone in the household depends on it, raise that at the start. It changes the design, rather than adding to it.

How Long Does Battery Backup Last in a Blackout ?

There is no single answer in hours. Any figure offered without knowledge of your household and your wiring is a guess. Runtime is the usable energy in the battery, set against the draw of whatever sits on the backed-up circuits.

The Difference Between Power and Energy

Two separate limits apply at the same time. They are easy to confuse, because manufacturers quote both in kilowatt units. The battery inverter must deliver the power to start pumps and motors at the moment they switch on. The battery itself must store the energy required over time.

	Power — kilowatts (kW)	Energy — kilowatt-hours (kWh)
Set by	The inverter.	The battery.
Answers	What can run at the same moment.	How long it can keep running.
Runs short when	Several motors or elements start together.	The outage lasts longer than the stored energy covers.
What you would notice	Backup trips, or refuses to start a pump or compressor.	Backup runs normally, then fades out.

This is why a large battery with a modest inverter can still fall short. The battery may hold plenty of stored energy. The inverter may still lack the instantaneous power to start a heat pump compressor and a bore pump at once.

Three things that move the runtime

How much energy is in the battery when the grid goes down. An outage at seven in the evening finds a very different state of charge from an outage at two in the afternoon.
How much energy you use while the grid is down. The backed-up circuits set the ceiling. Your own behaviour within that ceiling still moves the runtime considerably.
How much solar energy the panels generate while the grid is down. A system that keeps its panels working while islanded may recharge through the day. That can turn one evening of backup into something longer.

One further thing can change the first of those three. A virtual power plant is a scheme in which your retailer can draw on your battery in exchange for payments. If your battery is enrolled in one, the operator can discharge it at times of high grid demand. It may then hold less energy than you expect when an outage begins. Our info guide on what a virtual power plant is covers what you trade away in exchange for those payments.

Backup Generator Versus Batteries

The two solve different halves of the same problem. That is why some households have both. A battery is quiet and automatic, and the energy it holds limits it. Fuel alone limits a generator, but somebody has to be at home to start it.

A battery starts on its own, makes no noise, and needs no fuel deliveries. Its stored energy is finite. In a cool-climate winter, the solar recharge that would extend it is at its weakest.
A generator can keep running for as long as the fuel lasts. That is what makes it the usual answer for supply over several dull days.
Together, the battery covers short and common outages automatically, and the generator covers the rare long one. A household that leaves the grid entirely needs a backup generator to guarantee its energy supply.

Adding Backup to an Existing Solar System

Backup is a configuration decision, not an automatic feature of owning solar panels and a battery. An installer can install a system with no backup wiring at all. Adding backup later is its own piece of work. It needs new switchgear, a sub-board, and in some cases different inverter equipment.

Not every battery system offers backup, and some products are not compliant as grid-connect backup. Choose carefully. Not every battery inverter is powerful enough to back up all the appliances in a house. That is why the installer splits the switchboard into essential and non-essential loads.

Questions Worth Asking Before You Sign

Backup at all: does this design include backup, and which piece of equipment provides it — a multi-mode inverter, or a separate automatic transfer switch ?
Which circuits: exactly what goes on the critical loads panel, and can I see that list before the work starts ?
Power limit: what is the backup power rating of the inverter, and which of my appliances will not start on it ?
Runtime: with my own essential-loads draw, how long would this configuration last, and what assumptions does that answer rest on ?
Solar while islanded: can the panels keep charging the battery during a daytime outage, or does backup run on stored energy alone ?
Compliance: is this equipment approved for grid-connected backup, and what paperwork do you lodge with my distributor ?
Retrofit scope: if you add backup to a system I already have, what are you replacing rather than adding ?

Some existing installations need little more than a sub-board and a switch. Others need a different inverter. Only an inspection of your own switchboard will show which case applies to you.

Where the Cost Question Belongs

Whether a battery pays for itself is a different question from whether it can keep your lights on. Sales conversations often mix the two. For background on the economics, see our companion guide on whether to add a battery to your existing solar power system. It covers what changes on your bill, and what decides whether a retrofit is worth the money.

Rebates are a further moving part.

Governments set rebates, not installers. Amounts, eligibility, and closing dates have changed more than once.
Check the current criteria with the agency that administers the scheme, before a rebate figure carries your backup decision.

To do the groundwork yourself first, browse the full set of info guides, how-to guides, and free online tools.

Energy Assessments for Battery Backup Decisions

Whether backup is worth the wiring, and which circuits belong behind it, is a whole-of-system question rather than a product question. An independent energy assessment starts from your own usage data and your existing equipment, not from a catalogue. It can therefore say what a given backup configuration would cover in your home.

If you would rather have a professional turn all of this into a clear, costed plan for your home, find out more about how our Energy Assessments can help you.

Frequently Asked Questions

Does my solar system work during a power outage ? Not by itself. A grid-connected system shuts down when the grid fails, and it stays down until the grid returns or backup-specific hardware isolates the house.
Does a solar battery power my house in a blackout ? Only if the installation includes equipment that can disconnect the house from the grid first. A battery behind a standard grid-tied inverter shuts down along with everything else.
What is anti-islanding, in plain terms ? It is the safety function that makes an inverter stop supplying power when grid voltage disappears. No electricity then flows into lines that may be under repair.
Why does my solar shut off in a blackout even when the sun is out ? The state of the grid triggers the shutdown, not the weather. Bright sunshine makes no difference to it.
How does solar battery backup work once the grid fails ? The system detects the outage and isolates the home from the grid. It then supplies the backed-up circuits from the battery, and on some systems from the panels as well.
What is an automatic transfer switch ? It is the device that senses the loss of grid supply and switches the home to its own supply. It opens the connection to the grid, so no power can flow back into the lines.
What is changeover time in home battery backup ? It is the gap between the failure of the grid and the start of the backup supply. Product specifications commonly quote it in the tens of milliseconds, and it varies between models.
Is home battery backup the same as a UPS ? Usually not. The legal definition of a UPS (uninterrupted power supply) covers only a supply with no break at all. Home backup normally has a brief changeover gap, so suppliers call it an emergency power supply.
What is a critical loads panel in a solar battery system ? It is a small sub-board that carries the circuits chosen to stay live during an outage. The installer wires it separately, so the system can disconnect the rest of the house.
How do I decide between essential versus non-essential loads ? Start from what genuinely matters over a day or two without power: refrigeration, lighting, communications, and any medical equipment. Leave high-draw appliances such as ovens and dryers off the backed-up side.
What is the difference between whole home battery backup vs partial backup ? Partial backup powers a nominated set of circuits from a sub-board. Whole home backup carries the house as wired, which needs more battery capacity and heavier switchboard work.
How long does home battery backup last in a power outage ? As long as the usable energy in the battery divided by the draw of the backed-up circuits allows. It therefore depends on capacity, on which circuits are backed up, and on the state of charge when the outage began.
What is the difference between power and energy on a battery specification ? Power in kilowatts is what the inverter can deliver at one moment, and energy in kilowatt-hours is the total the battery can supply over time. Both limits apply at once.
Why will backup not start my heat pump even though the battery is full ? Motors and compressors draw a surge of power as they start. The limit you meet is the power rating of the inverter, not the stored energy in the battery.
Can solar recharge the battery during a blackout ? Some backup configurations keep the panels working while the home is islanded, which can extend runtime through daylight hours. Whether yours does is a question for your installer.
Can I add a backup battery for an existing solar system ? Often yes, but it is a distinct piece of work with switchboard changes and possibly different inverter equipment. Treat it as its own project rather than an accessory.
How do I find out whether my existing system already has backup ? Ask your installer or an independent assessor to confirm it from the equipment and the switchboard. The presence of a battery on its own does not settle the question.
Backup generator versus batteries — which should I consider ? A battery handles short, common outages automatically and quietly. A generator can run for as long as there is fuel, which is why long-outage resilience often combines the two.
Does backup capability affect my grid connection paperwork ? Equipment connected to a grid-tied system must be approved for that use and declared to your distributor. Ask what your installer lodges, and keep a copy.
Is backup worth the extra cost ? This is an economic question, not a technical one — it depends on how often your area loses power and what an outage would cost you. Our guide on adding a battery to an existing solar power system covers the money side.

Try our NEM12 Solar-Export Battery-Sizer Tool

Glossary

Anti-Islanding: The safety function that makes a grid-connected inverter stop supplying power when grid voltage is lost, so that electricity cannot flow into network lines that may be under repair.
Automatic Transfer Switch (ATS): A device that detects the loss of grid supply and switches a home over to an alternative supply, opening the connection to the network so power cannot flow back into it.
Changeover Time: The interval between the grid supply failing and a backup supply taking over the load; published as a product specification because it differs between models.
Critical Loads Panel: A separate sub-board carrying the circuits selected to remain powered during an outage, wired so the rest of the installation can be isolated. Also called an essential loads sub-board.
Emergency Power Supply (EPS): A supply that takes over after a short interruption when the normal supply fails — the arrangement most home battery backup uses.
Essential Loads: The appliances and circuits a household chooses to keep running during an outage, kept deliberately small so that the available stored energy lasts.
Grid-Connected Solar: A solar installation that operates in parallel with the mains electricity network, able to draw from it and export to it, as distinct from a standalone off-grid system.
Islanding: A condition in which part of the electricity network continues to be energised by a local generator while disconnected from the wider grid. It is dangerous when it happens by accident, and is the basis of backup when it is created deliberately and safely.
Kilowatt (kW): A unit of power — the rate at which electricity is being delivered or used at a given moment. Inverter ratings are given in kilowatts.
Kilowatt Hour: The unit of energy an electricity bill counts, written kWh. One kilowatt hour is the energy a 1,000-watt appliance uses in one hour.
Multi-Mode Inverter: An inverter able to operate both connected to the grid and, during an outage, as the supply for an isolated section of a home's wiring. Often described in quotes as a backup-capable or hybrid inverter.
State of Charge: How full a battery is at a given moment, usually expressed as a percentage of its usable capacity.
Uninterruptible Power Supply (UPS): A supply arrangement in which the connected load experiences no break at all when the normal supply fails — a stricter standard than most home battery backup meets.
Usable Capacity: The portion of a battery's rated energy storage that may actually be drawn on, which is less than the nominal figure because a reserve is held back to protect the cells.

Try our NEM12 Solar-Export Battery-Sizer Tool