Getting the sizing right for your solar and battery system is crucial. An undersized system leaves savings on the table, while an oversized one wastes money on capacity you don't use. This step-by-step guide will help you determine the right solar panel and battery size for your Australian home.
Step 1: Determine Your Daily Energy Usage
Your electricity bill is the starting point. Look for your daily average consumption, usually listed in kilowatt-hours (kWh) per day. If your bill shows quarterly usage, divide by the number of days in the billing period.
| Household Type | Typical Daily Usage | Quarterly Bill (approx) |
|---|---|---|
| 1–2 people, apartment/unit | 8–15 kWh | $250–$450 |
| 2–3 people, small house | 15–20 kWh | $400–$600 |
| 3–4 people, average house | 20–28 kWh | $550–$800 |
| 4–5 people, large house | 25–35 kWh | $700–$1,000 |
| 5+ people, large house with pool/EV | 35–50+ kWh | $900–$1,500+ |
Step 2: Size Your Solar to Cover Your Usage
As a general rule, your solar system should generate at least as much energy per day as you consume. Here's a rough guide:
| Daily Usage | Recommended Solar Size | Expected Daily Output |
|---|---|---|
| 10–15 kWh | 5–6.6 kW | 18–28 kWh |
| 15–25 kWh | 6.6–8 kW | 22–34 kWh |
| 25–35 kWh | 8–10 kW | 30–42 kWh |
| 35–50 kWh | 10–13 kW | 35–55 kWh |
Notice that the recommended solar size often exceeds daily usage. This is intentional — you want excess generation to charge your battery and account for cloudy days, seasonal variation, and panel degradation over time.
Location Adjustment
Solar output varies by location. A 6.6kW system produces more in Brisbane than in Hobart. Use these multipliers as a rough guide:
- Brisbane, Darwin, Perth: High output — 4.0–4.5 kWh per kW per day
- Sydney, Adelaide: Good output — 3.8–4.2 kWh per kW per day
- Melbourne, Canberra: Moderate output — 3.5–3.9 kWh per kW per day
- Hobart: Lower output — 3.2–3.6 kWh per kW per day
Step 3: Size Your Battery for Evening Usage
Your battery needs to cover the energy you use between sunset and sunrise — when your panels aren't generating. For most Australian households, this is roughly 40–60% of total daily usage.
| Daily Usage | Evening Usage (approx) | Recommended Battery |
|---|---|---|
| 15 kWh | 7–9 kWh | 5–10 kWh |
| 20 kWh | 9–12 kWh | 10 kWh |
| 25 kWh | 12–15 kWh | 10–13.5 kWh |
| 30 kWh | 14–18 kWh | 13.5–15 kWh |
| 40 kWh | 18–24 kWh | 15–20 kWh |
Common Sizing Mistakes
Mistake 1: Battery Too Big for Solar
If your solar system doesn't generate enough excess energy to fully charge your battery most days, you're wasting battery capacity. A 13.5kWh battery paired with a 5kW solar system on a house that uses most of the solar during the day may never fully charge. Ensure your solar system has enough excess to fill the battery after covering daytime loads.
Mistake 2: Ignoring Future Needs
If you're planning to buy an electric vehicle in the next few years, factor that in now. An EV adds 8–15 kWh of daily demand. It's cheaper to install a larger solar system upfront than to add panels later — but if you already have solar, our guide to adding a battery to an existing solar system covers retrofit options. Similarly, if you're planning a pool, heat pump, or home extension, size accordingly.
Mistake 3: Sizing for Peak Instead of Average
Your hottest summer day might use 50 kWh, but if your average is 25 kWh, don't size for 50. You'll have an oversized (and overpriced) system that sits partially idle most of the year. Size for your average usage and accept that extreme days will require some grid power.
Mistake 4: Forgetting About Export Limits
Many distribution networks limit solar export to 5kW on single-phase connections. If you install a 13kW system but can only export 5kW, excess generation above your consumption plus 5kW export is wasted. A battery can help by absorbing this excess, but factor this into your sizing calculations.
Mistake 5: Not Considering Self-Consumption First
Before adding a battery, maximise your solar self-consumption through load shifting. Running appliances during solar hours is free. If you can boost self-consumption from 30% to 50% through behaviour changes alone, you might need a smaller (or no) battery.
Get a Personalised Recommendation
Every household is different, and the right system size depends on your specific consumption pattern, location, roof space, and budget. Use our Solar ROI Calculator and Battery Payback Calculator to model different configurations and find the optimal balance of system size, cost, and return for your home.