How long will your battery last in a Perth blackout? A guide to backup duration

A 10kWh battery does not run your whole house for 10 hours. During a Perth blackout it powers whatever loads are switched on until it's depleted, and the arithmetic is simple: usable battery capacity in kWh, divided by your continuous load in kW, gives you the hours.

For a typical 10kWh battery, that works out to roughly 14 to 21 hours if you're running the fridge, lights and a few devices, but only 1.5 to 2.5 hours if a ducted air conditioner runs the whole time. Two things also have to be true before any of that backup time exists at all. Your system needs backup-capable hardware: a hybrid inverter with an EPS (Emergency Power Supply) mode, or an all-in-one battery with a backup gateway. And it needs a backup circuit wired in at install, an essential-loads sub-board or changeover device that lets part of the house island itself from the grid. A battery bolted onto a standard grid-tied inverter with neither of those does nothing when the power goes out.

Here's how to work out your own number.

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How do you calculate battery backup duration?

Backup duration = (battery usable capacity in kWh) ÷ (continuous load in kW)

Usable capacity is not the same as rated capacity. A fully charged 10kWh battery gives you roughly 8.6kWh once depth-of-discharge limits and inverter losses are accounted for, and the section below on usable capacity shows where that number comes from. Every example on this page uses it.

Example, very light load (no freezer):

• 10kWh battery = 8.6kWh usable
• Household running refrigerator (0.15kW) + LED lights (0.1kW) + router + phone chargers (0.05kW) = 0.3kW continuous load
• Duration: 8.6kWh ÷ 0.3kW = 28.7 hours

Example with air conditioning:

• Same 10kWh battery (8.6kWh usable)
• Add a ducted reverse-cycle AC at 3kW continuous
• New total load: 3.3kW
• Duration: 8.6kWh ÷ 3.3kW = 2.6 hours

Air conditioning changes backup duration by an order of magnitude. If you're still deciding what size battery to buy in the first place, how to size a home battery for Perth walks through matching capacity to your actual evening load, backup included.

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How much power do common Perth appliances draw?

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How much of your battery's rated capacity can you actually use?

The full rated capacity of a battery, say 10kWh, is not all usable during an outage. Three things eat into it.

Depth of discharge (DoD). Most batteries are warranted with a maximum DoD of 80–90%. A 10kWh LFP battery with 90% DoD has 9kWh usable. Push past that and you accelerate degradation, and you'll typically void the warranty in the process. Our battery warranty guide covers what those DoD limits actually mean for your cover.

State of charge at outage time. If the grid goes down at 8pm and your battery is at 50% charge (5kWh remaining on a 10kWh battery), you have 5kWh available, not 10kWh.

Inverter efficiency losses. The inverter converting battery DC to household AC runs at roughly 95–97% efficiency. A 10kWh battery delivers around 9.5–9.7kWh of usable AC energy.

Put those together and a 10kWh battery, fully charged when the outage starts, gives you approximately 8.6kWh of realistic usable capacity (10kWh × 90% DoD × 96% inverter efficiency). That 8.6kWh figure is what every worked example and scenario on this page is built on.

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What causes Perth blackouts, and does solar keep working through one?

Storms and equipment faults are common causes of outages in Perth. If the outage happens during daylight hours, a backup-capable system can keep drawing from solar as well as the battery, which extends how long you'll last.

Standard grid-connected solar systems shut down the moment the grid does. That's not a fault, it's a safety requirement (anti-islanding protection): the system has to stop feeding power out so lineworkers aren't at risk while they repair the fault. A system that keeps running through a blackout needs two things at once, hardware and wiring. The hardware is a hybrid inverter with an EPS mode, or an all-in-one battery with a backup gateway. The wiring is a backup circuit, an essential-loads sub-board or changeover device that separates part of the house from the grid so the system can safely island itself. Hardware alone doesn't do it: without the wiring, the battery still can't power anything when the grid drops. Our guide to solar and battery blackout power in Perth covers this two-part requirement in more depth.

Not every hybrid inverter supports backup/EPS mode, and not every installation wires it in even when the hardware supports it. Confirm with your installer whether your specific inverter and battery combination is configured for backup. Sungrow SBH, BYD HVS/HVM, and GoodWe Lynx all support some form of backup, but exactly how much of the house it covers, and whether that's single-phase or three-phase, varies by product and by how it's installed. BYD is worth calling out separately: it makes batteries, not inverters, so a BYD stack only backs anything up once it's paired with a compatible three-phase hybrid from another brand. If you're weighing an Enphase system, our Enphase IQ Battery Perth guide covers what it can and can't do for backup.

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How long will your battery actually last, scenario by scenario?

All four scenarios below use the 8.6kWh usable-capacity figure from a fully charged 10kWh battery.

Basic comfort backup (no AC, no cooking):

• Refrigerator + freezer + lighting + devices: ~0.4–0.6kW
• 10kWh battery (8.6kWh usable): 14–21 hours

Medical equipment backup:

• CPAP: ~0.05–0.1kW
• Oxygen concentrator (if applicable): 0.3–0.6kW
• Refrigerator for medication: 0.15kW
• Lighting and devices: 0.15kW
• Total: ~0.7–1.0kW
• 10kWh battery: 8–12 hours

If medical equipment is essential and grid reliability is a concern, a separate UPS (Uninterruptible Power Supply) dedicated to that equipment is a more targeted solution than whole-home battery backup. A generator is another option worth weighing alongside a battery; battery vs generator for backup power in Perth compares the two on cost, run time, noise and what each actually protects.

Summer comfort backup (with split-system AC):

• Split-system AC (2.5kW, moderate operation): 0.8–1.2kW average
• Refrigerator + lighting + devices: 0.4kW
• Total: ~1.2–1.6kW
• 10kWh battery: 5–7 hours

Summer comfort backup (ducted AC):

• Ducted AC (large home): 3–5kW
• Other loads: 0.4kW
• Total: ~3.4–5.4kW
• 10kWh battery: 1.5–2.5 hours

In a Perth summer, running AC through an extended blackout is battery-intensive. If food safety, medication, or a fan for cooling matter more than air conditioning, prioritise those first and treat the AC as the thing you switch off.

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How do you extend your battery's backup duration?

Prioritise loads. During a blackout, turn off non-essential loads. Disconnecting the ducted AC and using a single fan instead extends backup from around 2 hours to 14 or more.

Recharging from solar during the outage. If your system is backup-capable, daytime sun keeps working for you: the panels can recharge the battery and offset some of your running load at the same time, on top of whatever charge the battery already had when the outage started. How much depends on cloud cover, panel orientation and how the inverter throttles output while it's islanded, so treat it as a bonus rather than something to plan around.

Thermal mass. A fridge holds a safe temperature for around 4 hours unopened. A full freezer holds it for 48 hours. Minimising how often you open either during a blackout cuts down the refrigeration power you actually need.

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For most Perth households, a 10kWh battery provides meaningful backup for essential loads (fridge, lights, devices) for 14 to 21 hours without air conditioning, and roughly 1.5 to 2.5 hours if a ducted air conditioner runs the whole time. Backup planning means deciding in advance which loads you'd actually prioritise if the grid went down tonight.