The honest answer: it depends on your electricity bill — not your roof size, not your neighbour's system, and not what a salesperson tells you.
Most Australians get sold a system based on what's popular or what's in stock. The right approach is to size the system around your actual daily usage. This guide explains exactly how to do that.
Why System Size Matters So Much
A system that's too small won't cover your usage — you'll still have a high electricity bill and a long payback period.
A system that's too big wastes money upfront. You'll generate more power than you use, export it to the grid for a tiny feed-in tariff (often as low as 4–6 cents per kWh), and take years longer to see a return.
Getting the size right is the single most important decision in going solar.
The Simple Formula Australian Installers Use
The starting point is your daily electricity usage in kWh, which is printed on every Australian electricity bill.
Once you know that number, the basic formula is:
Daily usage (kWh) ÷ peak sun hours for your state = solar system size (kW)
Peak sun hours vary by state:
| State | Average Peak Sun Hours |
|---|---|
| QLD | 5.2 hours |
| NSW | 4.8 hours |
| VIC | 4.4 hours |
| SA | 5.0 hours |
| WA | 5.4 hours |
| TAS | 3.8 hours |
| ACT | 4.6 hours |
| NT | 6.0 hours |
Example: A Sydney household using 28 kWh per day needs roughly a 6kW system to cover their daytime usage (28 ÷ 4.8 = 5.8kW, rounded up to 6kW).
But this is just the starting point. A proper sizing calculation also accounts for:
- Seasonal variation — you use more power in winter, but generate less solar
- Whether you have existing solar already on the roof
- Time of use — when during the day you're home and using power
- Battery storage — if you want to store power for use at night
- Roof orientation and shading — north-facing gets more sun than east or west
What Size System Do Most Australian Homes Need?
| Household | Daily Usage | Recommended System |
|---|---|---|
| 1–2 people, small unit | 8–12 kWh/day | 3kW – 5kW |
| 3–4 people, average home | 15–25 kWh/day | 6kW – 10kW |
| 4–5 people, large home | 25–35 kWh/day | 10kW – 15kW |
| Large home with pool, EV | 35–50+ kWh/day | 15kW – 20kW+ |
These are averages. The actual number for your home could be quite different depending on whether you work from home, run air conditioning heavily, have an electric hot water system, or charge an electric vehicle.
This is why your electricity bill is the only reliable starting point. It has your actual usage, not an estimate.
How to Find Your Daily Usage on Your Bill
Every Australian electricity bill shows your usage. Look for:
- Total kWh used for the billing period (usually 30, 60 or 90 days)
- Billing period — the number of days the bill covers
Divide total kWh by number of days to get your daily average.
For example: 2,556 kWh over 90 days = 28.4 kWh per day
Some retailers print the daily average directly on the bill. AGL, Origin and Energy Australia all show this clearly. If yours doesn't, the maths is straightforward.
Don't want to do the maths yourself?
Upload your electricity bill and SolarBill reads your usage automatically — then calculates the right system size for your home in 60 seconds.
Get My System Size Free ☀️The Seasonal Problem Most People Miss
Here's where most solar sizing goes wrong.
If you size your system based on a summer bill when you're running air conditioning all day, your system will be oversized for the rest of the year.
If you size based on a spring bill when usage is low, you'll be undersized for winter when heating costs are highest and solar generation is lowest.
The right approach is to collect bills from different seasons — ideally one from each quarter — and size the system around your winter usage, when the demand is highest and solar production is lowest.
Most installers don't do this. They take one bill and run with it. SolarBill accounts for seasonal variation automatically — upload multiple bills and the calculator weights the recommendation towards your higher-usage periods.
Does Going Bigger Always Make Sense?
Not necessarily. There are two reasons to be cautious about oversizing:
1. Feed-in tariffs are very low
Most Australian states now offer feed-in tariffs of just 4–10 cents per kWh. That means power you export to the grid is worth very little compared to the 28–35 cents per kWh you pay to buy it back.
An oversized system generates a lot of exported power at low rates, which extends your payback period.
2. Network export limits
Many distribution networks in Australia now limit how much power you can export. In some areas this is as low as 5kW. A 15kW system on a limited export connection generates power that has nowhere to go during the middle of the day when you're not home.
Should You Add a Battery?
Adding a battery changes the equation significantly. With a battery you can store excess solar generation and use it at night — which is far more valuable than exporting it at 5 cents per kWh.
The federal Cheaper Home Batteries Program (launched July 2025) provides approximately $300 per kWh of battery capacity, which has made batteries genuinely cost-effective for most Australian households for the first time.
If you're considering a battery, you may want to size your solar system slightly larger than your daytime usage to ensure you're generating enough to fill the battery as well.
A common rule of thumb: add 2–3kW to your base solar size recommendation if you're adding a 10–13kWh battery.
Summary
- Size your system based on your actual daily kWh usage from your bill
- Divide daily usage by your state's peak sun hours to get a starting system size
- Account for seasonal variation — size for winter, not summer
- Consider battery storage if you're home in the evenings
- Don't oversize just to maximise exports — feed-in tariffs are too low
- Get a recommendation based on your actual bill, not industry averages
Find out your exact system size in 60 seconds
Upload your electricity bill and SolarBill calculates exactly what size system you need, your estimated savings and your payback period. Free, no account required.
Try the Free Calculator ☀️SolarBill is a free solar calculator for Australian homeowners. Works with AGL, Origin, Energy Australia, Alinta, EnergyAustralia and all major Australian retailers.