Calculating solar STCs takes four steps and about thirty seconds once you have the inputs: confirm the system size, find the zone from the postcode, apply the deeming years for the installation date and round down. The arithmetic is simple. The errors come from the inputs, so most of this article is about getting those right and building a check into your quote process.
If you want ready-made tables of results by system size, see how many STCs a solar system gets. This guide is the method behind the tables, with different worked examples, a spreadsheet formula you can drop into your quoting tool and the error checks we run when reviewing claims.
Step 1: confirm the system size in kW
The size is the total rated capacity of the panels, not the inverter. Multiply the number of panels by the rated watts and divide by 1,000.
- 20 panels x 440 W = 8,800 W = 8.8 kW
- 22 panels x 450 W = 9,900 W = 9.9 kW
- 10 panels x 440 W = 4,400 W = 4.4 kW
Use the rating on the approved product listing, not a marketing figure. The system must be under 100 kW to claim small-scale STCs (mid-scale solar up to 1 MW has its own rules from 1 October 2026; see mid-scale solar STCs).
Step 2: find the zone rating
The postcode sets the zone, and the zone sets the rating.
| Zone | Rating |
|---|---|
| 1 | 1.622 |
| 2 | 1.536 |
| 3 | 1.382 |
| 4 | 1.185 |
Look up the postcode, not the suburb or the nearest city. See the zone ratings guide and the answers on STC zone rating by postcode and the zone map.
Step 3: apply the deeming years
The deeming period depends on the year of installation: five years in 2026, four in 2027, three in 2028, two in 2029 and one in 2030. The scheme ends on 31 December 2030.
Step 4: multiply and round down
STCs = kW x zone rating x deeming years, rounded down to a whole number.
| Example | Calculation | STCs |
|---|---|---|
| 4.4 kW, zone 4, 2026 | 4.4 x 1.185 x 5 = 26.07 | 26 |
| 8.8 kW, zone 2, 2026 | 8.8 x 1.536 x 5 = 67.58 | 67 |
| 9.9 kW, zone 3, 2026 | 9.9 x 1.382 x 5 = 68.41 | 68 |
| 8.8 kW, zone 3, 2027 | 8.8 x 1.382 x 4 = 48.65 | 48 |
Turn it into dollars
Multiply the count by the rate you can lock. At the time of writing, spot has been roughly $38 to $40, and the clearing house pays $40 ex GST. At $38, the 9.9 kW zone 3 job is 68 x $38 = $2,584. At $40 it is $2,720. Today’s published rate is on our pricing page, and our guide to what an STC is worth covers the market.
A spreadsheet that does it for you
Set up four columns: kW, zone rating, deeming years, price. Then:
- STC count:
=ROUNDDOWN(A2*B2*C2,0) - Dollar value:
=E2*D2 - A zone lookup: store postcode ranges in a separate sheet and use
VLOOKUPorXLOOKUPso the rating is filled from the postcode. - A year lookup: derive the deeming years from the planned installation date, for example
=MAX(1,2031-YEAR(date))for dates between 2026 and 2030.
The last formula returns 5 for 2026, 4 for 2027 and so on, down to 1 for 2030. Restrict the date input so nobody types a year outside the scheme.
Checks before you quote
| Check | Common error |
|---|---|
| kW from panels, not inverter | Using inverter size |
| Zone from postcode | Using the zone of the nearest city |
| Year from installation date | Using the contract date |
| Round down | Rounding to nearest |
| New, addition or replacement | Counting the whole system when only part qualifies |
| Under 100 kW, or up to 1 MW under the mid-scale rules | Claiming STCs on a larger system without checking the rules |
Additions and replacements
If you are adding panels to an existing system, only the added capacity earns certificates, and the capacity added may be subject to the rules on the installation type. If you are replacing panels, certificates may not apply, or may apply to a smaller amount. Choose the installation type first and calculate second; the guide on installation type sets out the categories, and the answer on adding panels to an existing system covers the quote.
Using calculators sensibly
Online calculators are fine for a first look, but they can be out of date on the deeming year, they may use a suburb-level zone lookup, and they often show unrounded results. The Clean Energy Regulator’s own calculator is the reference. Run the final figure through it on the actual installation date, and save the result in the job file. If the number on the quote and the number on the claim differ, you should be able to explain why. See also the STC calculator for solar.
Batteries, heat pumps and hot water
Batteries and hot water use different formulas. Battery STCs depend on usable capacity and a tiered factor; see how to calculate battery STCs. Heat pumps and solar hot water depend on the model’s regulator-listed data; see heat pump hot water STCs explained.
Worked check: does the quote match the claim?
Suppose a quote says “6.6 kW system, 47 STCs, $1,786 discount at $38”. Run the four steps. If the postcode is zone 3 and the install is in 2026, the count is 45, not 47. Where did 47 come from? Possibly a zone 2 lookup (50) or a rounding-up of 45.6 to 46 and a typo. Whatever the cause, the claim will be for 45, and the customer has been promised a discount $76 larger than the certificates will pay (2 x $38). Multiply that across a year and the error becomes a margin leak.
The check takes under a minute: recompute from the four inputs, compare to the quote, and fix the template if they differ. Do it for the first five jobs each month until you trust the tool.
When the postcode sits on a boundary
A handful of postcodes sit across zone boundaries or have been reassigned in the regulator’s tables. If the lookup returns an unexpected zone for a regional postcode, check the regulator’s postcode table directly rather than trusting a third-party tool. The difference between zones 2 and 3 on a 9.9 kW system is about 8 STCs, around $300 at $38, which is worth a minute of checking.
What to do next
Add the formula to your quote template, run it against your last ten jobs and see whether any would have come out differently. Read the STC trading pillar for what happens after the count is created, and start trading when you have a claim ready. The pillar page on pricing shows how the count turns into cash.