Batteries guide
What size home battery do I need?
A battery is the right size when it stores what you'd otherwise buy from the grid in the evening and overnight, your solar can fill it on most days, and you're not paying for capacity you rarely use.
Work it out in four steps
Step 1: Find your evening and overnight use
This is what a battery replaces. A smart meter's interval data (ask your retailer, or check your retailer's app) shows how much you use from late afternoon until the sun's back on the panels.
Step 2: Use usable capacity
Compare batteries on usable kWh, not total or nominal kWh. Some energy is also lost charging and discharging.
Step 3: Check your solar can fill it
On an average day you need spare solar, beyond what the home uses, to charge the battery. Undersized solar leaves a big battery half full.
Step 4: Decide on backup
If you want power in a blackout, the battery's power output (kW) and which circuits it backs up matter as much as its capacity.
How a battery shifts solar into the evening
Daytime
Solar panelsGenerating
Your homeUses solar first
BatteryCharging
The gridGets what's left
Evening and night
BatteryDischarging
Your homeEvening peak
The gridBackstop
- Solar energy
- Battery energy
- Grid energy
How much spare solar fills each size
Spare solar capacity needed to fill the battery on an average day in zone 3 (South East Queensland and Sydney), allowing for 10% charging losses and real output at 85% of the Clean Energy Regulator's rating. Your system needs to be bigger than this to run the home during the day as well.
- 10 kWh usable: about 3.5 kW of spare solar
- 13.5 kWh usable: about 4.7 kW of spare solar
- 20 kWh usable: about 6.9 kW of spare solar
- 30 kWh usable: about 10.4 kW of spare solar
The federal discount by battery size
The Cheaper Home Batteries Program pays per kWh of usable capacity, at a lower rate for capacity above 14 kWh and again above 28 kWh. For batteries installed from 1 May 2026 to 31 December 2026, at 6.8 certificates per kWh:
- 10 kWh usable: 68 certificates, about $2,584
- 13.5 kWh usable: 91 certificates, about $3,458
- 20 kWh usable: 119 certificates, about $4,522
- 30 kWh usable: 154 certificates, about $5,852
- From 1 January 2027 the rate drops to 5.7 certificates per kWh.
- Above 14 kWh each extra kWh earns 60% of the full rate, and above 28 kWh, 15%. Only the first 50 kWh counts. Amounts use $38 a certificate; the market price varies.
Ask each installer
- How they sized the battery from your usage, and what share of your evening use it should cover
- The usable capacity, continuous power (kW) and peak power of the battery quoted
- Whether your solar can fill it on a typical winter day
- Which circuits stay on in a blackout, if backup is included
- Whether more modules can be added later, and whether that would earn the federal discount
Sources
Facts on this page were checked on 15 October 2026. Rules and programs change, so confirm details with your installer.
Frequently asked questions
Is a bigger battery always better?
No. A battery that rarely empties or rarely fills costs more without saving more. Size it to your evening and overnight use and to the spare solar you have.
Can I add more battery capacity later?
Many modular batteries can be expanded, often only with the same model and sometimes only within a set time after installation. The federal discount applies to one battery system per premises, so ask how an expansion would be treated.
How do I check the numbers for my home?
The battery rebate calculator uses your battery size, install date, solar and tariffs to estimate the discount, net cost and savings, without asking for any personal details.
Get battery quotes sized for your home
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