What size battery for a 13 kW solar system?
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Short answer: size the battery to what your home uses from late afternoon to the next morning, not to the 13 kW of panels. A 13 kW system makes about 45–57 kWh a day averaged over a year, depending on the city [SB-S219, E], and Australian homes use 11–23 kWh a day in total [SB-S220, C]. So most 13 kW homes have far more spare solar than they can store. Your night use sets the size. The federal discount pays the full rate only on the first 14 kWh, then less [SB-S036, SB-S001, C], so going past 28 kWh earns little extra discount.
On this page: What 13 kW makes / How big a battery / Worked example / How many modules / Big batteries and the discount / Winter / Ask your installer
What a 13 kW system makes
Averaged over a year, 1 kW of panels makes about 3.5–5 kWh a day in Australia [SB-S219, C]. Times 13:
| City | Typical a day from 13 kW (yearly average) |
|---|---|
| Perth | about 57.2 kWh |
| Brisbane / Adelaide | about 54.6 kWh |
| Sydney | about 52.0 kWh |
| Melbourne | about 46.8 kWh |
| Hobart | about 45.5 kWh |
Our arithmetic from energy.gov.au's per-kW figures [SB-S219, E]. Summer days make more and winter days less [SB-S219, C].
How big a battery for 13 kW?
- Find your night use. Your bill or smart-meter data shows daily use. Check a summer bill and a winter bill, because use changes with the season and time of day [SB-S305, C].
- Find your spare solar. Daily solar minus what you use while the sun is up [SB-S219, SB-S220, E].
- Pick the smaller of the two. A battery bigger than your night use won't empty. A battery bigger than your spare solar won't fill [SB-S219, SB-S220, E].
With 13 kW, step 2 is rarely the limit outside winter, so step 1 usually decides [SB-S219, SB-S220, E].
Worked example (one case, use your own numbers)
A Melbourne home makes about 46.8 kWh on an average day [SB-S219, E]. It uses 10 kWh while the sun is up, so about 36.8 kWh is spare [SB-S219, E]. Overnight it uses 15 kWh. The fit is about 15 kWh of usable storage, not 37 [SB-S219, SB-S220, E]. A bigger battery would sit part-full most nights.
When a bigger battery makes sense
- You use a lot at night, for example heating, a pool pump or an EV, and your bill shows it [SB-S305, E].
- You want blackout backup. That needs a battery and inverter set up for backup, and runtime depends on what you run [SB-S303, C].
- You plan to add load soon. Ask whether the battery can take extra modules later, rather than buying it all now [SB-S219, E].
How many modules?
Most home batteries are modular. Step sizes differ by brand: Sungrow SBR 3.2 kWh, FoxESS 4.66, Sigenergy 8.76, Alpha ESS 9.3, Powerwall 3 13.5 [SB-S137, SB-S054, SB-S060, SB-S074, SB-S052, C]. Pick the brand whose steps land closest to your number [SB-S219, E].
Big batteries and the discount
Above 14 kWh each extra kWh earns 60% of the full certificates, above 28 kWh only 15%, and nothing above 50 kWh [SB-S036, SB-S001, C]. On a 13 kW system it's tempting to go big; the discount is a reason to size to your real night use instead [SB-S036, E].
Winter
Your 13 kW system makes less in winter [SB-S219, C], but even in Hobart's yearly average there's about 45 kWh a day to share [SB-S219, E]. More panels make a 13 kW home more likely to fill its battery in July than a smaller system. We haven't found official winter figures for each roof, so ask your installer for a monthly estimate.
If your inverter is a few years old, it may need replacing or a second, battery inverter added [SB-S304, C]; read up on retrofitting a battery to an older system first.
Ask your installer
- Your spare solar in summer and in winter, from your own meter data [SB-S305, C].
- The battery's usable kWh and how many certificates the quote assumes for your install date [SB-S004, C].
- Whether your inverter can take a battery, or you need a hybrid or second inverter [SB-S304, C].
- Whether the battery is on the CEC approved list and VPP-capable [SB-S003, C].
The federal battery discount
The Cheaper Home Batteries Program gives a discount through certificates (STCs). Installs from May to December 2026 earn 6.8 certificates per usable kWh; from 1 January 2027 it's 5.7 [SB-S036, C]. The full rate covers the first 14 kWh, then 60% to 28 kWh and 15% to 50 kWh [SB-S036, SB-S001, C]. The rate is set by your installation date, not the day you sign [SB-S004, C]. The STC price moves, so there's no fixed dollar amount off. The battery must be on the Clean Energy Council approved list and be VPP-capable, and the installer must be accredited by Solar Accreditation Australia [SB-S003, C]. Check the current rules at cer.gov.au and dcceew.gov.au.
Other system sizes
- On a smaller roof? Read about sizing for a 6.6 kW system.
- Got 10 kW of panels? Follow the 10 kW sizing steps.
- Still deciding? You can compare all battery sizes on one page.
We haven't installed or used these batteries ourselves. This page is research from government pages, manufacturer documents and installer price lists, each linked and dated.
Rules checked 3 November 2026. Next scheduled check: before 1 January 2027 (STC step-down), and monthly for WA and NSW.