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To choose the right battery size, start by calculating your overnight electricity usage. Match the battery’s usable capacity to that figure, and ensure your solar system can generate enough energy to charge it fully. Most Australian homes with a 6.6 kW solar system suit a battery between 8 and 13 kWh.
How Much Energy Does Your Home Actually Use Overnight?
The first step in choosing the right battery size is understanding how much electricity your home consumes, especially after sunset. Your electricity bill is the best place to start. Look for your average daily usage measured in kilowatt-hours (kWh). In Australia, many households use between 15 and 25 kWh per day, although this varies depending on home size and lifestyle.
The key figure to focus on is how much energy you use overnight, from sunset to sunrise. As a general guide, many homes use roughly two-thirds of their total daily consumption outside of peak solar production hours. If your daily average is 20 kWh, you may be using around 13 kWh overnight. That overnight usage is the number your battery should ideally be able to cover.
What Does Battery Capacity Really Mean?
Battery size is measured in kilowatt-hours, such as 10 kWh or 13.5 kWh. However, there is an important distinction between total capacity and usable capacity. Batteries are not drained to zero because this would shorten their lifespan. A portion of the stored energy is reserved to protect the system.
For example, a battery with a 10 kWh total capacity may only provide 9 or 9.5 kWh of usable capacity. When comparing battery options, always focus on usable capacity, as this is the energy available to power your home. If your overnight usage is around 13 kWh, then a battery with usable capacity in the 10 to 13.5 kWh range is often a good match.
Does Your Solar System Support the Battery Size?
Battery sizing must align with your solar system size. There is little benefit in installing a large 15 kWh battery if your solar system cannot generate enough surplus energy during the day to fully charge it.
As a general rule, a solar system of at least 5 to 6 kilowatts is recommended before adding a battery. A common 6.6 kW solar system pairs well with a battery in the 8 to 13 kWh range. If your battery is oversized relative to your solar array, it may rarely reach full charge, reducing the financial value of the investment.
Should You Plan for Future Energy Needs?
When choosing battery size, it is also important to think ahead. Future additions such as an electric vehicle, pool equipment or increased air conditioning use can significantly raise your electricity consumption.
While a battery can reduce grid reliance, it is not a guarantee of zero electricity bills. Grid connection fees still apply, and cloudy winter days may prevent the battery from fully charging. Some homeowners prefer modular battery systems that allow additional capacity to be added later if energy usage increases.
Choosing the right battery size is about balancing current usage, solar generation capacity and future needs without overspending on unused storage.
