Fast read
For most small to medium Australian households, a battery with around 10 kWh to 13.5 kWh of usable capacity is an ideal starting point. This size typically stores enough excess solar energy to power a home through the expensive evening peak and overnight. The best size ultimately depends on nightly energy consumption and future goals, so choosing a modular system that can be expanded later offers the greatest flexibility and long-term value.
The High-Stakes Decision of Sizing Your Home Battery
Choosing the right size for a home battery is one of the most important decisions you’ll make on your renewable energy journey. It can feel daunting, with a significant investment on the line. Many homeowners worry about either overspending on capacity they’ll never use or undersizing their system and being left disappointed.
The good news is that finding the perfect fit isn’t about a magic number—it’s about understanding your own energy habits. This guide will walk you through the key considerations to confidently choose a battery size that perfectly matches your home, your lifestyle, and your goals.
It’s not about your house size, it’s about your energy use
The first and most critical point to understand is that the number of bedrooms in your house has very little to do with the right battery size. A retired couple in a four-bedroom home might use far less power than a young family of three in a smaller townhouse. The key metric is your average daily energy consumption, specifically what you use overnight.
Your electricity bill is the best place to start. Look for your ‘average daily usage’ in kilowatt-hours (kWh). As a general benchmark, data from the Australian Energy Regulator’s 2020 report shows a three-person household averages around 18.71 kWh per day, while a four-person household uses about 21.36 kWh per day. Of course, this varies significantly by location and lifestyle. For instance, a four-person household in Sydney might average around 20 kWh daily, while a similar home in colder Hobart could use considerably more. Typically, about 60-65% of that energy is used in the evening and overnight when your solar panels aren’t generating power.
This “after-dark” number is your starting point. If you use 20 kWh per day, you likely consume around 12-13 kWh after the sun goes down. This is the amount of energy your battery would need to store to get you through the night without drawing from the grid.
The golden rule: Matching your battery to your solar system
Your battery is charged by excess solar energy, so its size should be in harmony with your solar panel system’s capacity. While 6.6kW systems were once the standard, the average residential solar system size in Australia has been steadily climbing. Analysis of industry data showed the average system size installed in 2024 was between 9.7 kW and 9.9 kW. This trend reflects a clear move towards larger arrays, with systems over 10kW now representing a significant portion of new installations.
A well-accepted guideline is to aim for 1.5 to 2 kWh of battery capacity for every 1 kW of solar panels.
For a 9kW solar system, this would suggest a battery size in the range of 13.5kWh to 15kWh. For a 10kW solar system, this points towards a 15kWh to 20kWh battery.
This ratio helps ensure your solar system can comfortably charge your battery on a typical sunny day, without having a huge amount of excess capacity that you paid for but can’t fill.
What are your main goals for getting a battery?
Beyond simple maths, your personal reasons for wanting a battery will heavily influence the right size.
Goal 1: Maximise self-consumption and lower bills (most common)
This means storing your free daytime solar power to use during the evening peak-demand period, when electricity from the grid is most expensive. For this goal, a battery sized to cover your average evening usage is perfect. For most small-to-medium homes, a popular and effective choice is a battery with 10- 13.5 kWh of capacity.
Goal 2: Blackout protection
If you want to keep the lights and essential appliances on during a grid outage, you need to consider two things: capacity (kWh) and power output (kW). A 5- 7 kWh battery might run your lights, fridge, and Wi-Fi for a few hours. If you want to power more, for longer, a 10- 13.5 kWh battery provides better coverage. It’s crucial to discuss with your installer which specific circuits you want to back up so they can size the system appropriately.
Goal 3: Future-proofing and EV charging
If you’re planning to buy an electric vehicle (EV) in the next few years, it’s wise to factor that in. A typical EV requires a significant amount of energy to charge overnight. In this case, opting for a larger battery (14kWh+) or, even better, a modular system that you can expand later, is a very smart move.
Why ‘usable capacity’ is the number you need to know
When comparing batteries, you might see terms like ‘total capacity’ and ‘usable capacity’. The number that matters for your calculations is the usable capacity. This figure represents the actual amount of energy you can draw from the battery.
Manufacturers hold a small portion of the battery’s total capacity in reserve to protect the battery’s health and prolong its lifespan. Thankfully, reputable modern batteries, particularly those using Lithium Iron Phosphate (LFP) chemistry, now have a very high depth of discharge (DoD), meaning almost all of their advertised capacity is usable. For example, the Tesla Powerwall 3 has a usable capacity of 13.5 kWh.
The power of starting smart with a modular system
For many homeowners, the best approach is one that offers flexibility. Modular battery systems are designed like building blocks, allowing you to start with a capacity that meets your current needs and budget, and add more storage later if your lifestyle changes.
This is a financially savvy way to enter the battery market without overcommitting. For instance, you could install a system with an initial 8 kWh of storage and add another 5 kWh module a few years down the line when you buy an EV. Leading brands are excelling in this area. Sigenergy SigenStor offers stackable 5kWh and 8kWh modules, allowing for tailored sizing and future expansion up to 48kWh per unit. Sungrow’s SBR series uses click-together 3.2 kWh modules, making it very easy to scale up your storage capacity as your needs grow.
Common pitfalls to avoid when sizing your battery
Gross Oversizing: The most common mistake is buying a much larger battery than you need. You end up paying thousands for stored energy you rarely use, which can blow out the financial payback period.
Undersizing in Disappointment: A battery that’s too small will be full by early afternoon and empty before you’ve finished cooking dinner. This leads to continued reliance on the grid and a feeling that the investment wasn’t worthwhile.
Forgetting Power Output (kW): A battery’s capacity (kWh) tells you how much energy it holds, but its power rating (kW) tells you how many appliances it can run at once. A battery with a low power rating might not be able to run your air conditioner and your kettle at the same time, even if it has plenty of stored energy.
Key questions to ask your installer
A professional, accredited installer is your greatest asset. In early 2024, the national accreditation scheme for solar installers and designers transitioned to a new operator. On February 29, 2024, the Clean Energy Regulator (CER) approved Solar Accreditation Australia (SAA) as the new operator, and the role previously held by the Clean Energy Council (CEC) ceased. Following a transition period, all installers must now be accredited by SAA to be eligible to claim Small-scale Technology Certificates (STCs) on their behalf. A good installer should perform a detailed analysis of your energy usage. Be prepared to ask them:
- “How did you calculate this recommended size based on my electricity bills and energy habits?”
- “What is the battery’s usable capacity and its continuous power output?”
- “Is this system modular, and what would be the process and cost to expand it in the future?”
- “What are the full warranty terms, including for workmanship and capacity retention over 10 years?”
Your next step to energy independence
Choosing the right battery size is a balance of analysing your usage, understanding your goals, and planning for the future. For the average small or medium Australian home, a system around 10- 13.5 kWh is a fantastic and highly effective starting point. By prioritising a modular design from a quality brand, you give yourself the power to adapt as your energy life evolves.
This knowledge will allow you to have a confident conversation with an installer, ensuring you get a system that delivers value, security, and energy independence for years to come.
