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Choosing the right size battery is about finding the smartest option for your needs, not just the biggest. The ideal size depends on your daily energy use, your solar system's output, and your primary goal, whether it's saving money or ensuring backup power. For a typical home with a 6.6kW solar system, a battery with 10kWh to 13.5kWh of usable capacity is a common and effective choice.
The Critical Question: What Size Home Battery Do You Really Need?
Deciding to invest in a home battery is a major step towards energy independence, but it brings up one of the most common and critical questions we hear: “What size battery should I get?” It’s a valid concern. Choosing a battery that’s too small means you’ll still be drawing expensive power from the grid at night. Opting for one that’s too large means you’ve paid for storage capacity you may never use, extending your payback period.
The good news is that finding the right size isn’t a guessing game. It’s a logical process of matching a battery’s capacity to your household’s unique energy profile and your personal goals. This guide will walk you through the essential steps to help you make a confident and well-informed decision.
Why ‘bigger isn’t always better’ with battery sizing
It’s a common assumption that a bigger battery must be better, but that’s rarely the case in reality. The goal is to find the most cost-effective size that meets your specific needs. An oversized battery not only costs more upfront but may also never be fully utilised, meaning you’ve invested in technology that isn’t delivering a return. The sweet spot is a battery that can comfortably cover your nightly energy consumption without being excessively large.
The first step: understanding your energy consumption
Before you can size a battery, you need a clear picture of how much electricity your household uses, particularly outside of daylight hours. The unit to focus on is the kilowatt-hour (kWh).
Your electricity bill is the best place to start. Most retailers provide a graph or table showing your average daily usage in kWh. It’s crucial to treat national averages with caution as usage varies dramatically. The most recent detailed official data comes from the Australian Energy Regulator’s (AER) 2020 Residential Energy Consumption Benchmarks. This data shows significant variation by state due to climate differences; for instance, Victorian households have the lowest average annual usage (approx. 12.64 kWh/day), while Tasmania has the highest (approx. 23.61 kWh/day), largely due to heating needs. As of August 2023, the AER is no longer required to update these benchmarks, as the mandate for retailers to include them on bills has been removed.
The most important figure for battery sizing is your overnight consumption—the amount of energy you use from when your solar panels stop producing in the late afternoon until they start again the next morning. If your bill doesn’t provide this detail, a simple monitoring device (often installed with your solar system) can give you a precise breakdown of your usage patterns.
How does your solar PV system size impact your choice?
Your solar and battery systems are two halves of a whole. The battery can only store the excess energy your panels generate. A small solar array will struggle to completely fill a very large battery, especially on cloudy days or during winter.
A common rule of thumb is to ensure your solar system is large enough to both cover your daytime energy needs and fully charge your battery. While 6.6 kW systems remain popular, the average size of new residential installations is increasing significantly. According to industry analyst SunWiz, the average size of a solar array increased to 9.9 kW in 2024. This trend is expected to continue, with projections suggesting average sizes could reach 13- 15 kW in the coming years to accommodate higher household consumption from EVs and electric appliances.
What is your primary goal for getting a battery?
Homeowners typically invest in a battery for one of two main reasons, and your priority will influence the ideal size.
Maximising self-consumption and savings: This is the most common goal. You want to store your excess solar energy to use at night, minimising how much power you buy from the grid. For this, you need a battery large enough to cover your typical evening and overnight usage.
Blackout protection: If your main concern is keeping the lights on and essential appliances running during a grid outage, your focus will be on capacity and power output. You’ll need to identify your “critical loads”—such as the fridge, some lights, and internet modem—and calculate how much energy they would consume over your desired backup period (e.g., 4-6 hours).
Matching battery size to your goals and usage
Once you know your overnight usage and your goals, you can start looking at specific battery sizes. Battery capacity is measured in kilowatt-hours (kWh).
For an average family using 15-20 kWh per day, with about 8-12 kWh of that being consumed overnight, a battery with around 10-13 kWh of usable capacity would be a strong candidate. This would allow you to significantly reduce your reliance on the grid most nights of the year.
Many modern batteries are modular, allowing you to start with a smaller capacity and add more later if your needs change. For instance, the Sungrow SBR battery requires a minimum of three 3.2 kWh modules for a starting capacity of 9.6 kWh, and can be expanded up to 25.6 kWh, offering flexibility. Similarly, the Sigenergy SigenStor is a highly scalable system where a single unit can house between one and six battery modules, available in 5 kWh or 8 kWh sizes. This allows for tailored capacity, from a single 5 kWh module up to a total of 48 kWh (6 x 8 kWh modules) in one stack.
Don’t forget these critical factors
Beyond simple capacity, there are a few other key specifications to consider:
Usable capacity and Depth of Discharge (DoD): A battery’s total capacity and its usable capacity are not always the same. The DoD specifies the percentage of the battery’s total energy that can be safely used. While many modern Lithium Iron Phosphate (LFP) batteries are advertised with a 100% DoD, it is an industry best practice to limit regular discharging to 80-90% to reduce cell degradation and significantly extend the battery’s operational lifespan. Always check the usable capacity when comparing models.
Power output: Measured in kilowatts (kW), this is the rate at which a battery can deliver electricity. It determines how many appliances you can run simultaneously. A battery with a high kWh capacity but a low kW power rating might struggle to run a kettle and an air conditioner at the same time.
Future-proofing: Are you planning to buy an electric vehicle (EV) or install a heat pump in the next few years? These will significantly increase your electricity consumption, so it may be wise to choose a slightly larger or easily expandable battery system from the outset.
What are the next steps?
Choosing the right size battery is a balancing act between your energy habits, your solar production, your budget, and your future goals. By understanding your daily energy consumption and being clear on what you want to achieve, you can narrow down your options effectively.
The best way to get a perfectly sized system is to work with an accredited installer. They can perform a detailed analysis of your energy data and recommend a solution that delivers the best value and performance for your specific circumstances.
