If you are comparing quotes for a 6.6 kW system, here is the number that sits behind the STC discount.
The calculation
STCs = system size (kW) x zone rating x deeming years, rounded down. For a 2026 install, deeming is five years.
| Zone | Rating | STCs | Value at $38 to $40 |
|---|---|---|---|
| 1 | 1.622 | 53 | about $2,010 to $2,120 |
| 2 | 1.536 | 50 | about $1,900 to $2,000 |
| 3 | 1.382 | 45 | about $1,710 to $1,800 |
| 4 | 1.185 | 39 | about $1,480 to $1,560 |
Sydney is zone 3, so most Sydney buyers see the 45 STC figure; Melbourne is zone 4, where the same system earns about 39. You can look up yours with the postcode guide.
What the price assumption means
STC spot has been roughly $38 to $40, with the clearing house ceiling at $40, at the time of writing. Installers set their own assumed value, and many use a little under market to keep a margin. The number on your quote is a line item showing the STC count and the dollar discount. See how the rebate is paid upfront.
What changes in 2027
On 1 January 2027 the deeming period drops to four years. The same 6.6 kW system in zone 3 would earn about 36 STCs, a loss of nine certificates, worth around $340 to $360. The scheme finishes at the end of 2030. See what changes in January.
Comparing two quotes
When two installers quote the same 6.6 kW system, put the numbers side by side: system price, STC count, assumed value per STC, STC discount, net price. If one shows 45 STCs at $38 and the other 45 at $32, the second is either more conservative or taking a bigger margin on the certificates. Neither is wrong, but you should know which. Compare the net price, the product list and the warranty, not the headline discount alone.
What this means for you
Ask your installer to show the STC count, the value per STC and the net price. The numbers should be consistent with the table above. For background, see what an STC is, the resources hub and the pricing page for current STC market rates.