Almost every Perth household asks this once a battery enters the conversation. The answer is usually frustratingly vague. The honest response is that it depends. But not in a hand-waving way. Battery runtime comes down to a small number of factors. These can be estimated quite well once they are understood.
This guide explains the two numbers that decide how long a battery lasts. It covers what a typical Perth home actually draws. It shows how to make a rough estimate for your own household. It explains why a blackout in daylight behaves very differently to one at midnight.
The Two Numbers That Decide Runtime
Every battery has two specs that matter for backup. Confusing them is the most common source of disappointment.
- Usable capacity, measured in kilowatt hours (kWh). This is the size of the tank. It shows how much energy the battery holds. It decides how long the battery can keep going.
- Backup power output, measured in kilowatts (kW). This is the size of the tap. It shows how much the battery can deliver at any one moment. It decides how much can run at the same time.
A large battery with modest output cannot run a ducted air conditioner and an oven together. This is true no matter how much energy it stores. A battery with strong output but small capacity will run plenty at once. But it will not last long. Good backup design balances both against what the household actually wants to keep running.
What a Perth Home Actually Uses
Consumption varies enormously by household. As a general guide, a single-person home in Perth often uses somewhere around 10 to 15 kilowatt hours a day. A family of four commonly lands between 25 and 30. A larger household with several air conditioners can reach 35 to 45. A home with reverse-cycle air conditioning, a fridge, an electric hot water system and the usual modern devices sits around 25 kilowatt hours a day as a reasonable baseline.
Backup runtime is not calculated on that full daily figure. During an outage most households run far less than normal. They use lights, the fridge, Wi-Fi, phones and perhaps a fan or one room of air conditioning. Stripped back to essentials, a home might draw only a few hundred watts. This is why a battery can carry a household through an evening far more comfortably than the daily total suggests.
Essentials Only, or the Whole House?
How a system is wired makes a real difference. Backup is usually configured in one of two ways. Essential-circuit backup connects a chosen set of circuits. These typically include lights, power points, the fridge and the internet. The battery powers what matters and ignores heavy loads. Blackout backup powers the entire house. This is more convenient but drains the battery far faster if large appliances are used.
For most Perth homes, essential-circuit backup gives the best of both. It offers long runtime on the things that matter. It avoids a ducted air conditioner emptying the battery in a couple of hours. The decision is made at design stage. It is worth thinking about which circuits genuinely need to stay live.
A Simple Way to Estimate Your Runtime
The arithmetic is straightforward. Runtime in hours is roughly the usable capacity in kilowatt hours divided by the average draw in kilowatts. A home drawing an average of half a kilowatt from a battery holding 10 usable kilowatt hours will run for roughly 20 hours. Double the draw and the runtime halves.
The table below gives a feel for what different loads do to a battery holding around 10 usable kilowatt hours. Figures are indicative and vary by appliance.
| What you are running | Typical draw | Rough time from 10 kWh |
|---|---|---|
| Fridge only (averaged) | Around 0.1 kW | Several days |
| Lights, fridge, Wi-Fi and devices | Around 0.4 kW | Around 24 hours |
| The above plus a TV and small appliances | Around 0.7 kW | Around 14 hours |
| One room of reverse-cycle air conditioning | Around 1 kW | Around 10 hours |
| Electric oven while cooking | Around 2.5 kW | Around 4 hours |
| Ducted air conditioning | Around 4 kW | Around 2 to 3 hours |
These are averages for illustration. A fridge cycles on and off rather than drawing continuously. Air conditioners vary widely with the weather and the setting.
The Part People Forget: The Sun Comes Up
A battery is often imagined as a fixed tank that empties and then leaves the household in the dark. In a home with solar, that is not what happens. Provided the system is designed for it, the panels keep generating during an outage. They recharge the battery through the day.
This changes the picture completely for a longer outage. Rather than a single fixed runtime, the household settles into a daily rhythm. Solar powers the home and refills the battery through daylight. The battery carries the evening and overnight. In Perth, with its strong year-round sunshine, a well-designed system can keep essential circuits running through a multi-day outage almost indefinitely. This is as long as consumption stays sensible.
What Shortens Runtime
A few things pull runtime down faster than people expect. All of them are worth knowing before an outage rather than during one.
- Heating and cooling. Air conditioning and electric heating are by far the biggest loads in most homes. They will flatten a battery quickly.
- Anything with an element. Ovens, cooktops, kettles, clothes dryers and electric hot water all draw heavily while running.
- Charging an electric vehicle. A car draws more than the rest of the house combined. EV charging is normally excluded from backup.
- A reserve setting. Many systems hold back a percentage of the battery specifically for blackouts. This protects backup runtime but means that portion is not available for everyday savings.
Sizing a Battery for Real Backup
Because runtime depends on both capacity and output, sizing a battery for backup starts with a simple question. What does this household want to keep running, and for how long? From there the numbers follow. A home wanting lights, the fridge and the internet through an evening needs far less than one wanting air conditioning through a summer outage.
Positive Energy Solutions designs battery systems around that answer. We work across SigEnergy SigenStor, Tesla Powerwall 3, Sungrow SBH, Fronius Reserva, BYD Battery-Box Premium HVM and iStore. Several of these are modular. Capacity can be matched to the household rather than the household adjusting to the product. Every installation is completed by our accredited installers and signed off to Positive Energy’s completion standards. It is backed by a 10-year installation workmanship warranty and Positive Energy Protection, powered by Omnidian, monitoring from day one.
Frequently Asked Questions
How long will a 10 kWh battery run my house?
On essential circuits drawing around 0.4 kilowatts, roughly a day. Running air conditioning or an oven, only a few hours. The honest answer depends far more on what is switched on than on the battery alone.
Will my battery run my whole house in a blackout?
It can, if the system is designed and wired for blackout backup and the battery has enough output. Many Perth households choose essential-circuit backup instead. It makes the stored energy last far longer.
Does my solar keep working during a blackout?
With a suitably designed battery system, yes. The panels continue generating and recharge the battery through the day. This is what allows a home to ride out an outage lasting several days.
Can I run my air conditioner off a battery?
A single room unit is usually manageable for a few hours. Ducted air conditioning draws so much that it will empty a typical home battery in around two to three hours. It is rarely included in backup circuits.
Find Out What Your Home Would Actually Need
Runtime is not a mystery. It is specific to each household. The right way to answer it is to look at what a home uses. Decide what genuinely needs to keep running. Size the battery and backup circuits to match.
Talk to the local team about a battery sized for the way your home actually runs: positiveenergy.com.au/contact/
About the Author
Jason Wiktorowicz
Sales Manager, Positive Energy Solutions | Perth, WA
Jason has been at the centre of Western Australia’s energy transition for over a decade. He was Orchestration Lead on Project Symphony. That was WA’s largest virtual power plant pilot. He was Stream Lead on Project Encore. He worked directly with AEMO, Western Power and Energy Policy WA. He helped shape how distributed energy resources connect and operate on the SWIS. That work included developing and rolling out CSIP-AUS. It is now mandatory for all new WA grid connections. He also led Emergency Solar Management. It was the first program of its kind in WA. He led VPP for Schools. It provided direct market benefits through Supplementary Reserve Capacity. He led REBAR, the WA Residential Battery Rebate scheme.
His earlier career spanned Synergy, Infinite Energy and Curtin University’s Sustainability Policy Institute. He co-authored peer-reviewed research on net-zero urban precincts, solar integration and peer-to-peer energy trading. It was published in the journal Urban Planning in 2018.
At Positive Energy Solutions, Jason leads sales with a focus on system design accuracy and long-term performance. Positive Energy Solutions designs and installs solar and battery systems, EV chargers and heat pumps. We hold partner accreditations with Fronius, SigEnergy, iStore and SunPower.
10-year installation workmanship warranty. Positive Energy Protection, powered by Omnidian, performance monitoring included on all systems.
