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Calculating the right battery size for your solar system isn’t a one-size-fits-all task. It depends on how much electricity your household uses—especially at night—along with the size of your solar system, your goals for energy independence, and the usable capacity of the battery, which is shaped by its Depth of Discharge (DoD).
For a typical Australian home using around 20 kWh per day, a battery in the 10–15 kWh range is often suitable, but that’s a rough guide at best. To get it right, you’ll need to analyse your actual energy use and speak with an accredited installer.
How to calculate the right battery size for your solar setup
Adding a battery to your solar PV system can significantly reduce your reliance on the grid and help you make the most of your solar generation. But sizing it correctly is critical. A battery that’s too small won’t meet your evening or backup needs. One that’s too large may never be fully charged and could cost more than necessary.
This guide walks you through the key factors to consider, helping you find a battery size that suits your household’s energy use, solar output, and long-term goals.
Start with your electricity use
Your household’s electricity usage is the most important factor in sizing your battery. The goal is to understand how much electricity you typically use each day—and, more importantly, how much of that occurs after the sun goes down.
Review your electricity bills to find your average daily consumption in kilowatt-hours (kWh). If you have access to smart meter data, this will give you a more detailed view of when your energy use occurs across a 24-hour period.
In many Australian homes, night-time consumption makes up around 60–70% of daily energy use, although this varies depending on your work schedule, appliances, and seasonal habits. A battery is especially valuable if you regularly use significant energy after sunset, when your solar panels are no longer producing.
Can your solar system charge the battery?
The size of your solar PV system directly affects how much excess energy is available to charge your battery. Larger systems typically generate more surplus during the day, especially if your daytime consumption is modest.
If your system regularly exports significant energy to the grid, this indicates potential to redirect that surplus into battery storage. However, if you use most of your solar energy during the day, battery benefits may be limited unless you increase your generation capacity.
Some installers use a simple guideline of matching 1 to 1.5 kWh of battery capacity for every 1 kW of solar panel capacity. For example, a 6.6kW solar system might pair well with a 10kWh battery, assuming typical usage patterns. These are only rough starting points and should always be refined based on your actual consumption.
Understand Depth of Discharge (DoD)
A battery’s total storage capacity is not the same as its usable capacity. Depth of Discharge (DoD) refers to the proportion of the battery’s full capacity that can be used without shortening its lifespan.
For example, if you require 10 kWh of usable energy each night and your battery has a DoD of 90%, then you would need a nominal capacity of at least 11.1 kWh (10 ÷ 0.9). It’s important to check manufacturer specifications, as some batteries advertise total capacity rather than usable capacity.
Modern lithium-ion batteries—especially Lithium Iron Phosphate (LFP) types—often support a high DoD of 80–100%. These are preferred in Australian homes for their safety, longevity, and thermal stability. Common options include Sungrow, Sigenergy, and Tesla Powerwall, which all use LFP chemistry or similar high-performance technologies.
Consider your backup needs and autonomy goals
Autonomy refers to how long your battery can supply your home during periods without solar generation or in the event of a grid outage.
For grid-connected homes, one day of autonomy is often enough to cover night-time usage and short blackouts. In this case, your battery only needs to be large enough to store excess solar from the day for use overnight.
Off-grid homes require more detailed planning. Depending on location and weather patterns, off-grid systems often need 3–5 days of autonomy to provide reliable power through prolonged cloudy periods.
In both cases, it helps to define which appliances you want to run during outages. Covering only essential loads—like lights, refrigeration, internet, and medical equipment—requires far less storage than running your entire household as normal.
A simple battery sizing example
Here’s a straightforward method for estimating your required battery size:
- Let’s say your household uses 20 kWh per day on average.
- You estimate that 60% of this use occurs after dark—that’s 12 kWh of night-time consumption.
- You want 1 day of autonomy, so your battery needs to provide 12 kWh of usable energy.
- If the battery has a DoD of 90%, divide 12 by 0.90 = 13.33 kWh.
- Add 10% for system inefficiencies and losses: 13.33 × 1.1 = 14.66 kWh. In this example, a battery with 13–15 kWh of nominal capacity would be a good match.
Other important considerations
Budget and incentives: Battery systems are a major investment. While prices are gradually falling, most still cost between $800 and $1,500 per kWh installed. Government rebates and virtual power plant (VPP) programs in states like Victoria and South Australia can reduce upfront costs.
Future energy needs: Consider whether your energy demand is likely to increase. For instance, adding an electric vehicle, pool pump, or electrifying your heating and cooking may justify a larger or expandable battery system. Many modern batteries, including those from Sungrow and Sigenergy, are modular and can be scaled over time.
Space and placement: Batteries need adequate space, ventilation, and protection from extreme weather. Check if your chosen battery is rated for outdoor installation and ensure it complies with Australian Standards and your DNSP’s connection rules.
Solar system compatibility: A battery only performs well if your solar array can reliably generate enough surplus energy to charge it. Systems below 5kW may struggle to support larger batteries during winter or extended cloudy periods.
Export limits and local grid rules: Some distribution networks place limits on the size of systems you can connect or how much you can export. An accredited installer will help you navigate these restrictions and design a compliant, optimised system.
What’s typical in Australia?
The most common residential battery size in Australia falls between 10 and 15 kWh. This range suits a wide variety of households, particularly those with 6kW to 8kW solar systems. The Tesla Powerwall 2, for example, offers 13.5 kWh of usable storage and is widely used for its simplicity and all-in-one design.
However, every home is different. The “average” battery may not be the right one for you— tailoring your system to your usage and goals is far more effective than relying on benchmarks alone.
Conclusion: Get a system that fits your life
Sizing a solar battery isn’t about chasing the biggest number—it’s about finding a capacity that aligns with your energy usage, solar generation, budget, and lifestyle. A well-matched battery will improve your self-sufficiency, reduce your energy bills, and provide resilience during blackouts.
The best way to move forward is to gather your electricity bills, consider your household’s typical night-time usage, and talk to a Clean Energy Council-accredited installer. Your Energy Answers can help by connecting you with trusted local professionals for tailored advice and quotes. With the right guidance, your solar battery investment can deliver years of reliable performance and peace of mind.
