Almost every quote you receive will lead with a number in kilowatts — 6.6 kW, 10 kW, 13.2 kW. That number is the easiest thing for a salesperson to inflate and the hardest thing for a household to sanity-check. The good news is you can get very close to the right answer with one bill and ten minutes.
Start with your daily usage, not your bill total
Find the line on your electricity bill that shows average daily usage in kWh. Most Australian households land somewhere between 12 and 25 kWh a day. That single figure matters far more than the dollar total, because the dollar total moves with tariffs and seasons.
Work out how much of it happens in daylight
Solar only offsets what you use while the sun is up, unless you add a battery. Roughly:
- Nobody home 9–5 — expect 15–25% of usage to be self-consumed.
- Someone home most days — 30–40% is realistic.
- Pool pump, EV or ducted air on a timer — you can push past 50% by shifting loads into the middle of the day.
The rough sizing rule
In most of Australia, 1 kW of north-facing panels produces about 4 kWh a day averaged across the year — a bit more in Brisbane and Perth, a bit less in Hobart. So a 6.6 kW system makes roughly 26 kWh a day. That is comfortably more than a typical household uses, which is exactly the point: you want to cover daytime usage first and export the rest.
Why 6.6 kW keeps showing up
Most homes have a single-phase connection with a 5 kW inverter limit, and inverters are allowed to be oversized by a third. 6.6 kW of panels on a 5 kW inverter is the sweet spot that fits that rule — which is why it's the most quoted system in the country. If your network allows a larger export limit, or you have three-phase power, bigger systems open up.
When to go bigger anyway
- You're planning an EV in the next few years.
- You're considering a battery — oversized solar fills it faster in winter.
- Your roof has east and west faces, which spreads generation across the day but lowers peak output.
Panels are the cheapest part of the system per watt. Adding capacity at install time is far cheaper than coming back in three years to add more.