How to work out the right size
Find your evening and overnight use
From your smart meter data, add up what you buy from about 4pm to 7am on a typical day. See reading smart meter data.
Find your spare solar
Add up what you export through the middle of the day. Check a winter day as well as a summer one.
Take the smaller of the two
The battery only saves on what it can both charge from solar and discharge into your home.
Add room for backup or growth
Extra capacity helps in blackouts and if an EV or heat pump is coming.
How size affects the federal discount
| Battery | STCs | Discount up to | Discount per kWh |
|---|---|---|---|
| 10 kWh | 68 | $2,720 | $272 |
| 14 kWh | 95 | $3,800 | $271 |
| 20 kWh | 119 | $4,760 | $238 |
| 28 kWh | 152 | $6,080 | $217 |
| 40 kWh | 164 | $6,560 | $164 |
| 50 kWh | 174 | $6,960 | $139 |
The discount per kWh falls sharply above 14 kWh, so a very large battery is mostly paid for at full price. Make sure you will use the extra capacity.
Typical sizes
| Size | Usually suits | Solar to charge it |
|---|---|---|
| 5 to 10 kWh | Small households, low evening use | 5 to 6.6 kW |
| 10 to 15 kWh | Most family homes | 6.6 kW or more |
| 15 to 25 kWh | Large homes, electric heating, pools, EVs | 8 to 10 kW or more |
| Over 25 kWh | Very high use, whole-home backup, three-phase homes | 10 kW or more |
Common questions
Usually 10 to 15 kWh, if your evening use is high enough to empty it. A 6.6 kW system rarely has enough spare winter output to fill a much bigger battery.
It suits homes with high evening use and plenty of spare solar. The federal discount counts capacity above 14 kWh at only 60%, so make sure you will use it.
Download your half-hourly smart meter data from your retailer or distributor and add up what you used from about 4pm to 7am.
Sources
Rules and program details come from government sources. Prices are industry averages, named and dated where used. Last checked October 2026. Prices include GST unless we say otherwise.