A solar system earns its system size in kilowatts, multiplied by the zone rating for its postcode, multiplied by the deeming period in years, rounded down to a whole number. That is the entire formula. A 6.6 kW system in zone 3 installed in 2026 earns 45 STCs, and the same system in 2027 earns 36.
The numbers that move are the zone rating, which is fixed by postcode, and the deeming period, which falls by a year each January. Everything else is arithmetic. The tables below do it for the system sizes installers quote most often, so you can sanity-check a quote or a calculator in a few seconds.
The three inputs
System size. Use the total capacity of the panels in kW, not the inverter. Ten 440 W panels is 4.4 kW. The system must be under 100 kW to claim at the small-scale rules; from 1 October 2026 systems up to 1 MW can claim under the mid-scale rules.
Zone rating. Australia is split into four zones by solar irradiance. The ratings are 1.622 (zone 1, the sunniest, including much of central and northern Australia), 1.536 (zone 2), 1.382 (zone 3, covering most of the large coastal cities) and 1.185 (zone 4, the coolest, including Tasmania). Your postcode decides the zone. Our zone rating guide has the lookup.
Deeming period. Five years for 2026 installs, four for 2027, three for 2028, two for 2029 and one for 2030. The year is the installation date, not the contract date.
2026 installs (5-year deeming)
| System | Zone 1 | Zone 2 | Zone 3 | Zone 4 |
|---|---|---|---|---|
| 3 kW | 24 | 23 | 20 | 17 |
| 5 kW | 40 | 38 | 34 | 29 |
| 6.6 kW | 53 | 50 | 45 | 39 |
| 10 kW | 81 | 76 | 69 | 59 |
| 13.3 kW | 107 | 102 | 91 | 78 |
For example, 10 kW in zone 3: 10 x 1.382 x 5 = 69.1, so 69 STCs. At $38 per certificate that is $2,622.
2027 installs (4-year deeming)
| System | Zone 1 | Zone 2 | Zone 3 | Zone 4 |
|---|---|---|---|---|
| 3 kW | 19 | 18 | 16 | 14 |
| 5 kW | 32 | 30 | 27 | 23 |
| 6.6 kW | 42 | 40 | 36 | 31 |
| 10 kW | 64 | 61 | 55 | 47 |
| 13.3 kW | 86 | 81 | 73 | 63 |
The zone 3 6.6 kW system loses 9 STCs on 1 January 2027. At $38 that is about $342 less discount. For a customer comparing a quote signed in December against a January install, the date is worth more than a negotiated discount.
From certificates to dollars
Multiply the count by the price you can actually lock. At the time of writing the spot market has been roughly $38 to $40 and the clearing house pays a fixed $40 ex GST. For a 6.6 kW zone 3 system:
- 45 STCs at $38 = $1,710
- 45 STCs at $40 = $1,800
Your actual return is the rate your buyer publishes on the day your claim is complete. See the pricing page for ours, and what an STC is worth in 2026 for the market context.
Common calculation mistakes
- Using inverter size. An 8 kW inverter on 6.6 kW of panels is a 6.6 kW system for STC purposes.
- Wrong zone. Border postcodes can be split between zones. Check the postcode, not the town.
- Wrong year. A job signed in 2026 but installed after 1 January 2027 earns the 2027 count.
- Rounding up. Round down. An STC calculator that shows 45.6 means 45.
- Counting battery capacity. Batteries have their own STC formula. See how to calculate battery STCs.
Additions and replacements
An extension to an existing system earns STCs only on the added capacity, and a replacement may earn nothing, depending on what is being swapped. The category you pick in the registry drives the result. The guide on installation type explains it, and the answer on adding panels to an existing system covers the quoting side.
A quick quote check you can do in your head
For a fast sanity check at the kitchen table, use these shortcuts for 2026 installs:
- Zone 3: STCs are roughly kW x 6.9. A 6.6 kW system is about 45.6, so 45.
- Zone 1: roughly kW x 8.1.
- Zone 2: roughly kW x 7.7.
- Zone 4: roughly kW x 5.9.
For 2027, multiply the zone rating by four instead of five: zone 3 becomes about 5.5 per kW. These are only checks. The number you put on a claim must come from the regulator’s own calculation, because a rounding difference of one certificate between your quote and the registry is enough to confuse a customer who has read a different number.
Why the count matters for margin
Certificate value is often the single biggest line a customer sees as a discount, so a one-STC error on a quote is small, but a systematic error is not. If your sales team uses zone 3 for every postcode, you will overstate the discount in zone 4 by about 15 per cent and understate it in zone 1 by a similar margin. Over a year of installs, that is a margin problem hiding in a spreadsheet. Build the postcode lookup into the quoting tool, and review the deeming year every 1 January.
Larger systems and the 100 kW line
Systems up to 100 kW can create STCs, and from 1 October 2026 so can systems above 100 kW and up to 1 MW, with a fixed five-year deeming period (see mid-scale solar STCs). Above 1 MW you would look at LGCs, a different certificate with its own accreditation. The answer on commercial solar above 100 kW explains where the line falls, and the commercial solar STC limit covers the details of the 100 kW cap.
Why zone and deeming year matter to margin
Take a business that quotes 300 solar jobs a year at an average 45 STCs. An error of a single deeming year on, say, 30 of those jobs after 1 January costs about 9 STCs each, or roughly $340 at $38, which is over $10,000 across the batch. Small structural errors in quoting tools scale quickly, and they are invisible until someone compares quotes with claims. Reconcile once a quarter.
What to do next
Use the STC trading pillar for the installer overview, and calculate STCs for solar for a step-by-step method with a calculator workflow. The answer on STCs for a 6.6 kW system is the short version. When the certificates are created, start trading.