You do not need a special tool to work out your solar rebate. The maths is three multiplications and a rounding step, and doing it yourself is the quickest way to check a quote. This page gives you the method. For the same query from a different angle, see the Australia-specific calculator guide.
The formula
STCs = system size (kW) x zone rating x deeming years, rounded down.
Then multiply by the STC price to get the dollar discount.
- System size is the rated capacity of the panels, not the inverter. Ten 440 W panels is 4.4 kW.
- Zone rating comes from your postcode: 1.622, 1.536, 1.382 or 1.185.
- Deeming years are the years left in the scheme at install. It is 5 for 2026, 4 for 2027, then 3, 2 and 1 for 2028 to 2030. The scheme ends 31 December 2030.
A worked example
You are installing 8.8 kW in zone 3 in 2026.
8.8 x 1.382 x 5 = 60.8, rounded down to 60 STCs.
At $38 each that is $2,280. At $40 it is $2,400. At the time of writing the market has been roughly $38 to $40.
The same system installed in 2027 gives 8.8 x 1.382 x 4 = 48.6, so 48 STCs. That is twelve fewer certificates.
Three common mistakes
- Using the inverter size instead of the panel capacity.
- Forgetting to round down. Fractions of an STC are not paid.
- Using the wrong year. The date of installation is what counts.
From the desk: if your quote has a lower figure than your calculation, the difference is usually the installer’s margin for risk and admin, or an older STC price. A small gap is normal. A large one deserves a question.
Checking against your quote
Once you have your number, compare it with the STC count on the quote. A match within a certificate or two means the installer has used the same zone and year. A bigger difference usually means they have assumed a later install date, a different zone or a smaller system size. Ask which, and ask for the working in writing.
What this means for you
Run the numbers before you accept a quote, and check them again if your installation date moves. The 6.6 kW worked table saves you the arithmetic for the most common size, and the deeming period and zone ratings guide has the postcode lookup. For wider reading, start at the resources hub or see STC trading to learn how installers turn certificates into cash.