Getting the battery size right is the single most important decision in a solar battery purchase. Too small and you exhaust the battery before midnight; too large and you're paying for capacity you never use. Neither is good value.
Start with your evening load
The purpose of a home battery is to cover your electricity use after the sun goes down — from roughly 4 pm to 10 pm for most households. This is the "evening load" your battery needs to serve.
A typical Australian household uses between 2 and 5 kWh in the evening. If you have electric heating or cooling that runs in winter evenings, it can be significantly more.
To estimate your evening load, look at your electricity bill's daily usage figure, then consider that roughly 40–60% of household consumption happens in the evening and morning.
Factor in your solar surplus
A battery can only store what your panels generate in excess of your daytime consumption. If you're home during the day and running appliances, your solar surplus — and therefore the amount available to store — will be smaller.
For a household exporting minimal solar (less than 3 kWh/day on average), a 5 kWh battery is likely sufficient. For a household exporting 8–12 kWh/day, a 10–15 kWh battery will capture most of the available surplus.
The right-sizing rule of thumb
A commonly used rule: size your battery at roughly 80% of your average daily solar surplus, capped at your evening load. This gives you a high likelihood of fully charging every day without paying for excess capacity.
For example:
- Average daily export: 8 kWh
- Evening load: 6 kWh
- Recommended size: ~6–7 kWh usable
Don't just buy the biggest battery you can afford
The temptation to "future-proof" by buying a large battery is understandable, but the economics rarely support it. Every kilowatt-hour of battery you buy costs money. If that capacity never gets used — because you don't have enough solar to fill it or enough load to drain it — it's dead capital.
The return on investment peaks at the point where the battery is consistently cycling (charging and discharging nearly fully) every day. That's the sweet spot to target.
Use a calculator, not a rule of thumb
The above rules of thumb are a starting point. Your actual optimal battery size depends on your specific tariff, solar system output, household load profile, location, and battery chemistry.
Our free battery sizing calculator runs a full hourly simulation based on your inputs and shows you the payback period and ROI for different battery sizes — so you can find the sweet spot for your exact situation.