On 1 January 2027 the deeming period for small-scale solar falls from five years to four. Every system installed from that date earns about a fifth fewer STCs than the identical system installed a day earlier. The zone ratings do not change, the STC price does not change because of it and the formula is the same. One number in the formula shrinks, and it shrinks for everyone.
This article is the arithmetic: the STC count for common system sizes in each of the four zones, before and after the change, and the one question that trips people up, which is which date counts. For the broader story of what else moves in January, see our companion piece on what changes in January.
The formula
For rooftop solar, the number of STCs is:
system size in kW x zone rating x deeming period in years, rounded down to a whole number.
The deeming period is the number of years from the install year through to the end of the scheme on 31 December 2030. For 2026 installs it is 5, for 2027 it is 4, then 3, 2 and finally 1 in 2030. The zone ratings are 1.622 for zone 1, 1.536 for zone 2, 1.382 for zone 3 and 1.185 for zone 4. The deeming period and zone ratings guide explains where your postcode sits.
STCs by zone and system size
Here is the count for the same system installed in 2026 (5 years) and in 2027 (4 years). Every figure is rounded down.
| System | Zone | 2026 STCs | 2027 STCs | Difference |
|---|---|---|---|---|
| 6.6 kW | 1 (1.622) | 53 | 42 | 11 |
| 6.6 kW | 2 (1.536) | 50 | 40 | 10 |
| 6.6 kW | 3 (1.382) | 45 | 36 | 9 |
| 6.6 kW | 4 (1.185) | 39 | 31 | 8 |
| 10 kW | 3 (1.382) | 69 | 55 | 14 |
| 13 kW | 3 (1.382) | 89 | 71 | 18 |
A worked line: 6.6 x 1.382 = 9.1212 per year. Five years gives 45.6, rounded down to 45. Four years gives 36.48, rounded down to 36. At $39 an STC, that is $1,755 against $1,404, a difference of $351 on a typical system. Use the STC calculator for 2027 if you want your own postcode.
What it means in dollars
At the time of writing, STC spot has been roughly $38 to $40, and the clearing house ceiling is $40. Using $39 as the midpoint:
- 6.6 kW zone 3: loses about $351 of discount.
- 10 kW zone 3: loses 14 STCs, about $546.
- A 5 kW system in zone 1 (5 x 1.622 x 5 = 40.55, so 40 STCs; 5 x 1.622 x 4 = 32.44, so 32): loses 8 STCs, about $312.
For a business doing 20 jobs a month at about 45 STCs, the January drop takes roughly 180 STCs a month out of the book. That is about $7,000 a month of certificate income at $39, which has to be recovered through margin, volume or price. The what is an STC worth in 2026 guide shows how to value what you hold.
Which date counts
Customers and installers both ask this, and the answer matters for pre-January contracts. The deeming period is set by the installation date of the system, not by the day the contract was signed or the day the quote was accepted. A job signed in November and installed in February earns the 2027 deeming period. A job installed on 30 December earns the 2026 one.
Two practical points follow:
- Installation and commissioning dates should be consistent across your photos, the CES or equivalent paperwork and the claim. A date mismatch is the sort of thing a regulator notices.
- Do not backdate. Putting an earlier install date on a system to capture the 5-year deeming period is a serious compliance breach. The photos, meter data and paperwork all carry dates, and the Clean Energy Regulator audits against them. See how STC audits work.
For the exact test, the Clean Energy Regulator’s guidance on the date of installation is the authority.
From the desk: if a customer wants “the 2026 rate” and the job genuinely cannot be installed and commissioned by 31 December, say so plainly. A crew that promises the older number and then installs in January has to either absorb the lost STCs or go back to the customer. Putting the likely STC count for both years on the quote, in writing, avoids an awkward conversation.
The schedule beyond 2027
The deeming period keeps falling by a year every January until 2030:
| Install year | Deeming years | 6.6 kW zone 3 STCs |
|---|---|---|
| 2026 | 5 | 45 |
| 2027 | 4 | 36 |
| 2028 | 3 | 27 |
| 2029 | 2 | 18 |
| 2030 | 1 | 9 |
The scheme ends on 31 December 2030. For the longer view, read the end of the scheme.
How installers can plan around January
- Schedule by install date, not sign date. Sort the pipeline and mark which jobs can realistically be energised before the end of December.
- Quote both numbers. A 2026 quote and a 2027 quote on the same system show the customer exactly what waiting costs.
- Hold your crews’ calendar to prioritise installs for customers who are close to the line, without cutting corners on photos or testing.
- Price 2027 on the 2027 formula. Do not carry a 2026 margin into January.
- Keep cash flowing. The January rush creates a lot of claims at once. A trader that settles within a day keeps your working capital steady. Our STC trading page explains how the claim flow works.
A note on quoting small jobs
The January change hits small systems proportionally the same as large ones, but the dollars look different on the quote. A 3 kW system in zone 3 earns 3 x 1.382 x 5 = 20.73, so 20 STCs in 2026, and 3 x 1.382 x 4 = 16.58, so 16 in 2027. At $39 that is $780 against $624, a drop of $156. On a small job priced tightly, that $156 can be the whole margin, so rerun your small-system pricing separately rather than assuming the large-system logic carries over. Rounding down also matters more on small systems, because losing a fraction of an STC is a larger share of a smaller number.
What to do next
- Run your January pipeline through the table above and mark jobs by install date.
- Update your quote template to show STCs and dollars for 2026 and 2027.
- Check today’s rate so you value certificates on a live number.
- Make sure assignment forms and photo sets are complete, since a rush is when mistakes happen. The photo requirements are the usual place to start.
- When you are ready to settle January’s claims faster, start trading.