Battery Backup in a Blackout: What Will Your Home Run?

The lights go out just as dinner starts. Your battery app showed plenty of charge an hour ago, so everything should keep working, right?

Possibly - but the answer depends on how the system was designed. A home battery can help keep essential appliances running through an outage.

It can also be installed mainly to shift solar energy into the evening, without providing the backup you expected.

Before choosing a battery installation, write down what you need to keep running. Then ask your installer to show how the proposed system will meet that list.

First, check whether backup is actually included

The Australian Government's Solar Consumer Guide explains that battery backup requires a suitable configuration.

Some systems supply selected circuits, some can support a whole property, and some installations on a three-phase property support only one phase. Solar recharging during an outage also depends on the system design.

Read the government's battery guide.

Ask for the backup scope in writing. “Battery included” does not explain whether the kitchen fridge, bedroom lights, garage door or home office sockets will have power.

A labelled circuit list is more useful than a broad promise about energy independence.

Solar Victoria also recommends making your backup requirements clear before installation, because the inverter, other components and electrical work all affect the result.

Solar Victoria's battery advice.

Separate what runs together from how long it runs

Two ratings matter. Kilowatt-hours, or kWh, describe stored energy. Kilowatts, or kW, describe power output - the amount the system can supply at a particular moment. Solar Victoria uses this distinction when explaining battery sizing.

How a solar battery system works.

Consider an illustrative backup system with 8 kWh available to deliver to appliances and a 5 kW continuous backup output. An average 0.5 kW load would suggest about 16 hours of operation.

At an average 2 kW, the same energy suggests about four hours.

Those are arithmetic examples, not promised runtimes. Real results depend on the energy available when the outage begins, operating losses, temperature, appliance cycles and any solar contribution.

The inverter may also have separate short-duration limits for starting equipment.

The 5 kW output figure presents a different constraint: adding more appliances can exceed what the system can supply simultaneously, even when plenty of energy remains.

Ask for the continuous backup rating and starting capability of the actual proposed installation.

Choose your essentials room by room

Start with an ordinary evening at home. Which rooms need light? Where does the fridge plug in? Does the internet connection need several powered devices? Is there a pump that matters to daily life?

• Food and communication: fridge, selected lights, phone charging, router and any required network equipment.
• Work and access: a chosen office outlet, laptop charging and garage-door access if included in the design.
• Larger discretionary loads: cooking appliances, space heating, cooling, pool equipment and EV charging.

These are discussion categories, not a guaranteed appliance list. A small air conditioner may be a reasonable priority for one home; a large ducted system may change another home's design substantially.

Check the actual equipment rather than relying on generic appliance wattage charts.

Tesla's Australian Powerwall guidance, for example, warns that starting energy-intensive loads during an outage can overload the system. That is a useful reminder to plan the sequence of appliance use as well as the total energy budget.

Tesla's outage guidance.

The battery percentage at the start matters

A large battery near empty can provide less useful backup than a smaller one with plenty of stored energy.

Discuss the reserve policy when the system is commissioned, including whether you can change it and what any retailer or virtual power plant arrangement permits.

For Powerwall 3 and the Powerwall range, Tesla describes Backup Reserve as energy set aside for outages, with the remaining portion available for normal operating modes.

A higher reserve trades some everyday energy use for a larger outage buffer. Other products have their own controls and limits.

Tesla's Backup Reserve explanation.

Ask the installer to demonstrate the relevant setting in your own app. Decide whether your priority is short interruptions, getting through an evening, or maintaining a small set of essentials during a longer event.

Those are different design briefs.

Can tomorrow's sunshine keep you going?

Only count on solar generation during an outage if the installer confirms that your particular combination of panels, inverter, battery and backup equipment supports it.

Also ask what happens after the battery reaches its minimum operating level: can it restart from solar, and under what conditions?

Even with that capability, plan for a cloudy winter day. Build an essentials-only scenario with no assumed solar recharge, then consider sunshine as a possible extension. That makes the plan easier to use when the weather is poor.

Ask for a practical handover

Before signing off, request a demonstration or installer-led test of the backup functions. Walk through the house together and confirm the agreed outlets and appliances.

Have the installer explain outage notifications, any brief interruption during transfer, and what to do if a load stops the backup supply.

Keep the circuit list and operating guide somewhere accessible without internet.

For equipment that cannot tolerate interruptions, ask its supplier about a dedicated backup arrangement; a household battery's changeover behaviour may not suit every device.

Contact Lightning Energy to assess your existing solar, switchboard and priority appliances, then help specify a battery installation around the backup you actually want.

Bring your electricity bill, inverter details and a short list of the things your household wants to keep running.