How to Choose the Right Solar System Size (Not the Biggest)
Bigger isn't always better — the right kW depends on how and when you actually use power, not on filling your roof.
Almost every solar quote you'll receive nudges you toward a bigger system. That's not always a con — panels are cheap per watt these days — but bigger only pays if you actually use or export the extra power at a decent price. The right size is the one that clears the most value off your bill for the least outlay, and that number is different for a shift-worker with a pool than for a couple who are out all day. Here's how to work it out from first principles.
Start with your usage, not your roof
The single most useful number is your daily kWh consumption — it's on your electricity bill, often shown as an average. Once you know that, you can estimate how much a given system will generate. As a national rule of thumb, rooftop solar produces roughly 1,400 kWh per kW of panels per year in Australia (your exact figure varies with location, roof pitch and shading — confirm for your address).
So a 6.6kW system generates around 9,240 kWh a year, or about 25 kWh on an average day — but that's spread across daylight hours, not delivered on demand at 7pm.
The real question isn't "how much will it generate?" It's "how much of that will I use myself versus export?" Self-consumed solar offsets what you'd otherwise buy; exported solar only earns the feed-in tariff. Those are very different values.
Why self-consumption beats export every time
When you use your own solar, you avoid paying the retail rate for grid power. When you export, you're paid the feed-in tariff — and that number has been falling. The typical range is now around 3 to 10 cents per kWh, and it's retailer-set in most states (VIC, regional QLD and TAS set a regulated minimum; confirm the current rate for your state).
Compare that to what you pay to buy power — usually several times higher. The maths is stark: every kWh you self-consume is worth far more than every kWh you export. That's why sizing is really about when you use power.
- Home during the day (retirees, remote workers, pool pump, day-run appliances) — a larger system pays, because you soak up more of it.
- Out all day, home at night — a modest system that covers your daytime baseload may give better payback than a big one dumping cheap exports to the grid.
- Willing to shift loads — running dishwasher, washing, hot water and EV charging in the middle of the day dramatically lifts self-consumption on a bigger system.
The export limit you might not know about
Here's a catch that catches people out. Many single-phase homes have an export limit of 5kW imposed by the distributor. You can often install more panels than that, but the amount you're allowed to push to the grid is capped. If you're on single-phase, oversizing to 10kW+ while being out all day means a chunk of that generation is either curtailed or exported at a few cents — a poor return on the extra panels.
This is exactly where independent advice matters: the answer depends on your phase supply, your distributor's rules and your usage pattern, none of which a generic quote accounts for.
What the panels actually cost
Prices already include the federal STC discount (the Small-scale Renewable Energy Scheme, applied at the point of sale). Indicative installed ranges:
| System size | Indicative price after STCs |
|---|---|
| 6.6 kW | $4,000 – $6,500 |
| 10 kW | $6,000 – $10,000 |
| 13.2 kW | $7,500 – $13,000 |
Notice the pattern: going from 6.6kW to 10kW adds roughly 50% more capacity for often less than 50% more cost. Panels have genuine economies of scale. That's the real argument for going bigger — if you can use or valuably export the extra output. If you can't, you're paying more for power you'll sell at 3–10c.
Where a bigger system genuinely makes sense
Oversizing isn't always waste. It can be a smart move if you're planning to:
- Add a battery later (the federal Cheaper Home Batteries program offers an STC-based discount of roughly 30% on usable kWh — larger solar keeps a battery charged).
- Buy an EV and charge it during the day.
- Electrify — e.g. a heat-pump hot water system (which also earns STCs) running on daytime solar.
- Take advantage of the federal Solar Sharer free-power window (11am–2pm, up to 24 kWh/day) on participating retailers — though note that's about cheap grid power, so it doesn't need your own panels.
The honest version: the biggest system might not be your best payback. If you're out every day, on single-phase with a 5kW export cap, and have no battery or EV plans, a right-sized 6.6kW can beat a 13.2kW system on return per dollar spent — because the extra panels earn only export cents.
We don't sell or install a single panel, so we've got no reason to talk you into a bigger system. We charge a small fixed fee to look at your actual bill, your usage pattern and your phase supply, and tell you the kW that pays best for you — even if that's smaller than the quotes in your inbox.
Want to know what actually pays for your place?
Get an independent, engineered read on your whole bill — solar, battery, tariff and the rebates you're really entitled to — for a small fixed fee. No system to sell, no sales call.
Start your audit — $100Figures in this guide are current at the date of publication and indicative only — rebate and certificate values change over time (federal battery certificates step down each January and July), and state schemes are amended often. Confirm the current numbers for your situation before you commit. This is general information, not personal financial advice.