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The Right Question to Ask First
Most people approach battery sizing from the wrong direction. They ask "how big a battery can I afford?" or "what's the most popular size?" The right question is simpler: how much electricity do you use in the evening and overnight?
A home battery's job is to capture surplus solar energy generated during the day and discharge it when the sun is not shining — primarily in the late afternoon and evening. Size the battery to match that evening load and you will have the right answer. Go significantly larger and you are paying for capacity that rarely fills up. Go significantly smaller and you will still draw from the grid each evening despite having a battery.
Step 1: Find Your Evening Usage
Your electricity retailer can provide half-hourly or 15-minute interval data for your property (if you have a smart meter). This shows exactly how much electricity you use at each time of day. Look at the period from roughly 5pm to 11pm — that is your target window for battery discharge.
If you do not have interval data, a reasonable estimate is that the average Australian household uses 40 to 50% of its daily electricity in the evening and overnight (after 5pm). If your daily total is 20 kWh, evening and overnight usage is roughly 8 to 10 kWh. That is your starting target for battery size.
Quick Sizing Table
| Household | Daily total usage | Evening usage (est.) | Recommended battery |
|---|---|---|---|
| 1–2 person, small home | 8–14 kWh | 3–6 kWh | 5–7 kWh |
| 2–3 person, average home | 14–20 kWh | 6–9 kWh | 10 kWh |
| 3–4 person, average home | 20–28 kWh | 8–12 kWh | 10–13.5 kWh |
| Large home or EV charging overnight | 28–45 kWh | 12–20 kWh | 15–20 kWh |
Step 2: Add the Backup Buffer (If You Want It)
If backup power during blackouts is important to you, you need to decide how many hours of essential power you want and for which loads. Essential loads typically include: fridge and freezer (100 to 200W continuous), lights (20 to 50W per room), phone and device charging (50 to 100W), and possibly a broadband router (20W).
Running these essentials draws roughly 300 to 500W continuously. At that rate, a 10kWh battery provides 20 to 33 hours of backup for essentials only. Adding air conditioning, electric cooking, or hot water changes the calculation significantly — each of these can draw 1,000 to 3,500W, which would drain a 10kWh battery in 3 to 10 hours.
For most households, a 10kWh battery sized for daily self-consumption also provides adequate blackout backup for essentials. Unless you have a medical device requiring continuous power or live in an area with extended outages, oversizing purely for backup rarely makes financial sense.
Step 3: Check Against Your Solar System Size
Your solar system needs to generate enough surplus to reliably fill the battery. A 6.6kW system generating 22 to 28 kWh per day in a typical Australian location, paired with daytime household usage of 8 to 12 kWh, leaves 10 to 16 kWh of surplus available to charge a battery. This comfortably fills a 10kWh battery on most days.
In winter, days are shorter and generation drops. A 6.6kW system in Melbourne might generate only 14 to 18 kWh on winter days. If daytime household usage is 8 kWh, only 6 to 10 kWh of surplus is available. A 10kWh battery may not fully charge on overcast winter days, which is acceptable — a partial charge still reduces your evening grid draw significantly.
The rough guideline: pair a 10kWh battery with a 6.6kW or larger solar system. A 5kWh battery can work with a 5kW system. For a 15kWh or larger battery, a 10kW system is the sensible minimum.
Popular Battery Options by Size
| Battery | Usable capacity | Installed price (approx.) | After CHB rebate |
|---|---|---|---|
| Sungrow SBR064 (6.4kWh) | 6.1 kWh | $7,000–$9,000 | ~$5,000–$6,500 |
| Sungrow SBR128 (12.8kWh) | 12.2 kWh | $11,000–$14,000 | ~$8,000–$10,000 |
| Tesla Powerwall 3 (13.5kWh) | 13.5 kWh | $14,000–$17,000 | ~$10,000–$12,500 |
| BYD Battery-Box HVS 10.2 | 10.2 kWh | $9,000–$12,000 | ~$6,500–$8,500 |
| GoodWe Lynx Home F 10.0 | 9.8 kWh | $8,500–$11,000 | ~$6,000–$8,000 |
For most Australian households, a 10kWh to 13.5kWh battery is the right range. The 10kWh covers average evening usage, provides meaningful blackout backup, and pairs well with a standard 6.6kW solar system. Going above 15kWh is justified for large households, EV overnight charging, or properties with extended outage risk.
The next step
If you have any questions about the information in this guide, feel free to get in touch:
Email: hello@whysolar.com.au
Tel: +61 433 405 530
If you're considering solar panels or batteries for your home, Jos and the team can help you get quotes from trusted, pre-vetted local installers:
Written by
Jos AguiarSolar Evangelist
Passionate about making solar simple and accessible for every Australian household. Jos breaks down complex energy topics into practical advice so homeowners can make confident decisions about solar, batteries, and energy independence.
Learn more about Jos Aguiar