The STC formula is system kW x zone rating x deeming years, rounded down. The only thing that changes on 1 January 2027 is the number of deeming years, from 5 to 4. Everything else in the formula stays, including the zone ratings of 1.622, 1.536, 1.382 and 1.185.
The drop in numbers
| System | Zone | 2026 (5 years) | 2027 (4 years) | STCs lost | Dollar loss at $38 to $40 |
|---|---|---|---|---|---|
| 6.6 kW | 3 (Sydney, Brisbane, Perth, Adelaide) | 45 | 36 | 9 | about $340 to $360 |
| 6.6 kW | 4 (Melbourne, Hobart) | 39 | 31 | 8 | about $300 to $320 |
| 10 kW | 3 | 69 | 55 | 14 | about $530 to $560 |
| 13 kW | 4 | 77 | 61 | 16 | about $610 to $640 |
For commercial work the loss is bigger. A 90 kW system in zone 3 earns 621 STCs in 2026 and 497 in 2027, a loss of 124 STCs or about $4,700 to $5,000.
What the 4-year deeming period costs in context
On a typical household system the 2027 loss is a few hundred dollars: noticeable, but modest next to the price of the system. Where a quote is advertised as “$X after rebate”, most of the difference between a December and a January install is these certificates. Waiting a year, with the 3-year period in 2028, cuts a total of about 40% from the 2026 level.
The dollar value moves independently
All the dollar figures use the STC spot range at the time of writing, roughly $38 to $40, with a clearing-house ceiling of $40. The market price can change in either direction, and the price you are paid depends on your agent or trader. Check the pricing page for a current published rate.
Systems above 100 kW
From 1 October 2026 systems above 100 kW and up to 1 MW create STCs with a fixed five-year deeming period, so the 2027 reduction does not touch them. See the mid-scale solar STCs pillar.
Should you rush
Rushing an install to beat 1 January saves about a fifth of the STCs but can cost more if it means skipping design, approvals or a good installer. Treat the date as one factor, and do not trade quality for a few hundred dollars.
What this means for installers
Put the deeming year on every quote, price jobs for December realistically and tell customers where the date risk sits. If you hold certificates from 2026 installs, these are already created and do not change. See the whole timeline, the 2027 deeming insight and the STC calculator. For settlement, see STC trading and start trading.