Checking your STC discount takes about a minute once you know the formula. This page covers the two linked questions: how much the discount on your quote should be, and how to check it or use a calculator.
The formula
STCs = system size (kW) x zone rating x deeming years, rounded down to a whole number.
- Zone ratings: 1.622 (zone 1), 1.536 (zone 2), 1.382 (zone 3), 1.185 (zone 4).
- Deeming years: 5 for systems installed in 2026, 4 for 2027, falling to 1 in 2030. The scheme ends on 31 December 2030.
- Zone is set by postcode in the Clean Energy Regulator’s table. Sydney, Brisbane, Perth, Adelaide, Canberra, Gold Coast and Newcastle are zone 3. Melbourne, Geelong, Hobart and Launceston are zone 4. Darwin is zone 2 and Alice Springs zone 1.
Worked examples at the time of writing
| System | Zone | STCs | Value at $38 to $40 |
|---|---|---|---|
| 6.6 kW, Sydney | 3 | 45 | $1,710 to $1,800 |
| 6.6 kW, Melbourne | 4 | 39 | $1,482 to $1,560 |
| 10 kW, Brisbane | 3 | 69 | $2,622 to $2,760 |
The values are estimates. The STC market has traded roughly between $38 and $40, with $40 the clearing house ceiling, and installers normally use a slightly lower figure.
Checking the number on your quote
- Find the system size on the quote, using the panel total in kW, not the inverter size.
- Look up your postcode zone with the STC calculator.
- Compare the calculator’s STC count with the count on the quote. They should match or be within one or two certificates.
- Look at the dollar value per STC and ask how it was set. Our page on the price installers use explains the normal gap.
If the count is lower than the calculator, ask for the installer’s working. A common innocent reason is a different deeming year or a system that was changed before installation. For installer-side numbers on a standard 6.6 kW job, see how much solar rebate a 6.6 kW system gets.
What this means for you
Use the calculator before you sign, keep the output, and compare it with each quote. If installers disagree by hundreds of dollars, the difference is usually the value per STC rather than the count. For why that value differs from the market, see the STC discount versus the spot price. The homeowners guide is a good next stop, and installers can compare certificate rates on the pricing page.