The number of STCs a solar system creates is not a negotiation. It is a calculation set by regulation, using three inputs: the size of the system, the zone it is installed in and the deeming period remaining. Understanding it helps you quote accurately, explain the January drop to customers and check a claim before you lodge it.
Figures below are those at the time of writing. Check the current factors with the Clean Energy Regulator before you quote.
The deeming period
Small-scale solar certificates are “deemed” up front. Instead of waiting for the system to generate power, the scheme credits the expected generation over the remaining life of the scheme in one go. That life is the deeming period.
At the time of writing, the scheme is legislated to run to 2030, and the deeming period reduces by one year at the start of each calendar year.
| Install year | Deeming period (years) |
|---|---|
| 2026 | 5 |
| 2027 | 4 |
| 2028 | 3 |
| 2029 | 2 |
| 2030 | 1 |
The year that counts is the installation year, so timing matters at year end.
The four zones
Australia is split into four zones by postcode, reflecting how much sun a system is expected to receive. Each zone has a rating, which is the expected generation per kW per year.
| Zone | Rating |
|---|---|
| Zone 1 | 1.622 |
| Zone 2 | 1.536 |
| Zone 3 | 1.382 |
| Zone 4 | 1.185 |
Zone 1 is the sunniest and Zone 4 the least. Your postcode decides the zone, not the nearest town.
The formula
The core calculation is:
System size (kW) × zone rating × deeming period (years) = STCs
The result is rounded down to a whole number. Any customer-facing quote should say it is an estimate until the system is installed and the claim is complete.
Worked example
A 6.6 kW system installed in 2026 in a Zone 3 postcode:
- System size: 6.6 kW
- Zone 3 rating: 1.382
- Deeming period in 2026: 5 years
- 6.6 × 1.382 × 5 = 45.606
- Rounded down: 45 STCs
The same system installed in 2027 has a four-year deeming period:
- 6.6 × 1.382 × 4 = 36.48
- Rounded down: 36 STCs
That is nine fewer certificates for the same system, which is why customers and installers feel the January change.
Why the count falls every January
Each 1 January, the deeming period drops by one year. The fall is built into the scheme, not a market move, so you can predict it. Practical consequences:
- Quotes written in late December and installed in January may carry fewer certificates than the customer expected.
- Jobs that straddle year end need a clear record of the real installation date.
- Pricing conversations with customers should treat the discount as a declining figure, year after year.
The certificate price is separate. It moves with the market, and today’s rate is on the pricing page. The count per job is fixed by formula. The value per certificate is the part that moves.
Check your numbers
Use the Clean Energy Regulator’s small-scale calculator to confirm a count before you quote. It uses the postcode and system size and applies the right factors for the date. If your figure differs from ours, ask the desk. It is nearly always a postcode, a size or a date.
Checklist for quoting
- Confirm the postcode zone
- Use the installed kW, not the nominal panel count
- Apply the deeming period for the installation year
- Round down
- State clearly the figure is an estimate
- Keep the working in the job file
Where this fits
Zone and size are also recorded on your paperwork, so errors repeat. Check the assignment form guide and the photo requirements checklist so the details agree everywhere. For the full solar process see the STC trading page, for the other certificate types see battery STCs and hot water STCs, and when you are ready to sell, start trading.