There are two things people mean by “STCs falling”: the number of certificates your system creates, and what each one sells for. The first follows a published timetable. The second does not.
The count: a fixed step down every January
STCs created by a solar system equal kilowatts of capacity, times your zone rating, times the deeming period in years. The deeming period is set by installation year and steps down by one year each 1 January until the scheme ends.
Here is a 6.6 kW system in zone 3 (Sydney, Brisbane, Perth, Adelaide, Canberra) using the zone rating of 1.382, with the count rounded down as the regulator does:
| Install year | Deeming period | STCs created | Change on 2026 |
|---|---|---|---|
| 2026 | 5 years | 45 | n/a |
| 2027 | 4 years | 36 | down 20% |
| 2028 | 3 years | 27 | down 40% |
| 2029 | 2 years | 18 | down 60% |
| 2030 | 1 year | 9 | down 80% |
Melbourne, Geelong and Hobart are zone 4 with a rating of 1.185, so the same system makes 39, 31, 23, 15 and 7. Darwin is zone 2 and Alice Springs zone 1, with higher ratings and so higher counts.
What that means in dollars
At the time of writing STCs have traded at roughly $38 to $40, and the clearing house price is capped at $40. On the zone 3 example, 45 STCs is about $1,700 to $1,800 before any installer margin. The same system in 2027 is about $1,370 to $1,440, in 2028 about $1,030 to $1,080, and by 2030 about $340 to $360. These are illustrations at today’s market, not forecasts.
How much will STCs fall in 2027, 2028, 2029 and 2030?
- 2027: down one fifth from 2026. The deeming period goes from 5 to 4 years.
- 2028: down a quarter from 2027 (4 to 3 years), or 40 percent below 2026.
- 2029: down a third from 2028 (3 to 2 years).
- 2030: down a half from 2029 (2 to 1 year). The scheme then closes on 31 December 2030.
The steepest percentage cuts come last, but in dollar terms every step takes roughly the same amount off a given system, because each step removes one year of deemed output.
The price is the wildcard
STC prices rise when supply gets tight and fall when it is plentiful. A shrinking count per system does not automatically lift the price, because market volume is driven by how many systems are installed, which has been buoyant. We would not plan a purchase around a price move in either direction.
What this means for you
If you are buying, calculate your own count and weigh it against the timing advice in should I wait for solar prices or STCs to drop. The legislated change is explained in STC deeming period 2027 and the deeming period and zone ratings guide. If you install, pricing shows today’s settlement rate and how it works shows how a claim moves.