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Matching your solar and battery size is a balancing act that starts with understanding your home's energy use, which you can find on your power bill. A good rule of thumb is to choose a battery with a usable capacity that can store the excess solar energy your system generates on a typical day, ensuring you have enough power for evenings and cloudy periods. For most Australian homes, a common and effective pairing is a 6.6kW solar system with a battery around 10–13.5kWh, but your ideal setup depends on your specific consumption patterns and future energy goals.
How do I match the size of my solar system to my battery size?
Deciding on the right solar and battery combination for your home is one of the most important steps in your renewable energy journey. It can feel like a complex puzzle, but getting the balance right is key to maximising your energy independence and savings. Think of your solar panels and your battery as a team: the panels are the daytime earners, and the battery is the savvy saver, storing energy for when you need it most. This guide will walk you through how to size that team perfectly for your household.
Why getting the size right matters
Choosing the right size for your solar and battery system is crucial for both performance and cost-effectiveness.
If your battery is too small for your solar array, you won’t be able to store all the excess energy your panels generate. This means you’ll still be exporting a significant amount of power to the grid for a diminishing feed-in tariff and then buying it back in the evening at a much higher rate.
On the other hand, if your battery is too large and your solar system isn’t big enough to fill it, you’ve paid for storage capacity that you can’t use. This leads to a higher upfront cost without the corresponding benefit, extending the time it takes for your investment to pay for itself. The goal is to find that sweet spot where your system meets your needs without unnecessary expense.
Start with your energy consumption
The first and most important step is to understand how much electricity your household uses. The best place to find this information is on your electricity bill, which usually shows your average daily usage in kilowatt-hours (kWh).
Data from the Australian Energy Regulator shows that a typical three-person household uses around 18.7 kWh per day, though this is a general average. Consumption varies significantly by location; for instance, a home in Victoria might average around 20 kWh per day, while a household in Tasmania, with higher heating demands, could use considerably more.
Once you have your average daily usage, consider when you use that energy. A typical family might use about a third of their energy during the day and the remaining two-thirds in the evening and overnight. This evening block is what your battery will primarily cover.
Sizing your solar PV system
With your energy usage clear, you can determine a suitable solar system size. The aim is for your panels to generate enough electricity to power your home during the day and fully charge your battery with the excess energy.
A very common residential solar system size in Australia is 6.6 kW. On a sunny day, this system can produce between 24 kWh and 33 kWh, depending on factors like your location, panel orientation, and weather. For example, a 6.6kW system in Melbourne might generate 23.8 kWh, while the same system in sunny Brisbane could produce 27.7 kWh. This is often more than enough to cover the average household’s daytime use and leave a surplus for battery charging.
However, the physical size of your roof and the amount of unshaded, sunny space available can be a limiting factor. An accredited installer can help you assess your roof’s potential.
Connecting the two: Sizing your battery
Once you know how much excess energy your solar system is likely to produce, you can choose a battery to store it. The key is to match the battery’s ‘usable capacity’ (the actual amount of energy you can get out of it) to your needs.
A popular rule of thumb suggests that for a 5 kW solar system, a battery with 10 kWh to 13.5 kWh of storage is often a good match to capture the excess power that would otherwise be exported.
For a 6.6kW solar system, a battery in the 10kWh to 15kWh range is often a great starting point for the average household. This allows you to store most of your excess solar generation, significantly reducing your reliance on the grid in the evenings. Many modern batteries are modular, meaning you can start with a smaller capacity, like 10 kWh, and add more storage later if your needs change.
What about AC vs DC coupling?
When installing a battery, you’ll encounter the terms ‘AC-coupled’ and ‘DC-coupled’. This refers to how the battery connects to your solar system and can influence efficiency.
DC-coupled systems connect the battery directly to the solar panels through a single hybrid inverter. This is highly efficient, with a round-trip efficiency of up to 98%, because the DC power from the panels goes straight into the DC battery without being converted. This setup is often more cost-effective for new solar and battery installations.
AC-coupled systems use a separate inverter for the battery, in addition to the solar inverter. The DC solar power is first converted to AC for your home, and any excess AC power is then converted back to DC to be stored in the battery. This double conversion results in slightly lower efficiency (90-94%) but offers more flexibility, especially when retrofitting a battery to an existing solar system.
For most new installations aiming for maximum efficiency, a DC-coupled system is generally the preferred path.
Key questions to ask your installer
To ensure you get the perfect match, have a conversation with your SAA-accredited installer. As of 29 February 2024, Solar Accreditation Australia (SAA) is the official body for accrediting solar designers and installers, a requirement for accessing government incentives like Small-scale Technology Certificates (STCs).
Here are some crucial questions to ask: Based on my energy bills and roof space, what solar and battery size do you recommend? Will you be proposing an AC or DC-coupled system, and why is that the best choice for my home? Can the battery system be expanded in the future if my energy needs increase? What is the battery’s usable capacity and depth of discharge (DoD)? How much power can the battery provide at once (its power rating in kW), and will it run all my essential appliances?
A final word on getting it right
Matching your solar system to your battery isn’t about finding a single magic number, but about creating a balanced system tailored to your unique circumstances. By understanding your energy consumption, considering your future needs, and working with a trusted professional, you can invest in a system that delivers clean energy, reduces your power bills, and provides energy security for years to come.
