How the calculator works
Each kilowatt of panels makes a predictable amount of power a year, depending on where you live. In Adelaide it’s about 1,500 kWh per kW on a north-facing roof, and less in Melbourne or Hobart. East and west roofs make about 14% less, and south-facing roofs about a third less.
The savings depend on how much of that power you use yourself. Power you use directly saves you the full price you pay (often 30 to 40 cents a kWh). Power you send to the grid earns only the feed-in tariff, which is often 5 cents or less. That’s why the share of power you use during the day matters, and why a battery can help: it stores daytime surplus for the evening.
How many panels fit on a roof?
A typical 440 W panel is about 1.95 m². Allowing for edges, vents and gaps, about three-quarters of a roof face can usually take panels. A 6.6 kW system (15 panels) needs roughly 40 m² of suitable roof.
Is a battery worth it?
A battery pays off when you have spare daytime power and evening use to soak it up, and when the price after rebates is low enough. Try a few sizes above and compare the payback times. Batteries don’t usually pay back as fast as panels, but rebates have improved the numbers.
Before you buy
- Most networks limit how much you can export. Check with your distributor before choosing the inverter size.
- Use an accredited installer so you can claim the small-scale technology certificates (STCs) that reduce the price, and any battery rebate.
- Panels add load to the roof and must be fixed for your site’s wind. That’s normally fine on a house in good condition, but check older roofs.
Common questions
How many solar panels do I need for my house?
Roughly enough to cover your daily use: divide your daily kWh by about 4 (in sunny parts of Australia) to get the kW, and by 0.44 to get the number of 440 W panels. 20 kWh a day is about 5 kW, or 11 to 12 panels. The calculator above refines this for your roof direction and location.
How long does solar take to pay for itself?
Commonly 3 to 6 years for panels in Australia, depending on how much power you use during the day, what you pay for electricity and the installed price. Batteries usually take longer.
Does a west-facing roof work for solar?
Yes. East and west roofs make about 14% less in total than north, but west panels produce more in the late afternoon, when many households use more power.
What size inverter do I need?
Usually a little smaller than the panels, as panels rarely produce their full rating. Your network’s export limits also matter, so check them before you buy.