Every installer knows STCs fall over time. Fewer know exactly how, and when it bites. This article sets out the mechanism for solar PV, shows the effect on a typical job, and gives you a practical plan for the year boundary.
As always, confirm figures with the Clean Energy Regulator (CER) for your installation date.
How the deeming period works
When you install a rooftop solar system, the scheme does not wait to see how much electricity it generates. It deems the output in advance, across the years remaining until the scheme ends in 2030.
The number of STCs for a system is, broadly:
system size (kW) x zone rating x deeming period (years), rounded down
The zone rating reflects how much sun the postcode gets. The deeming period is the number of years left in the scheme. As the end date stays fixed at 2030, the period shrinks by one year each 1 January.
For small-scale solar, the pattern runs:
| Installation year | Deeming period |
|---|---|
| 2026 | 5 years |
| 2027 | 4 years |
| 2028 | 3 years |
| 2029 | 2 years |
| 2030 | 1 year |
Details of zone ratings are in our deeming period and zone ratings explainer.
What changes on 1 January 2027
On 1 January 2027, the deeming period drops from 5 years to 4. Every system installed from that date creates about one fifth fewer certificates than the identical system installed in December 2026.
Worked example
This uses the Zone 3 rating of 1.382, which covers much of Sydney, Melbourne’s northern edge and large parts of regional Australia. Use the rating for your own postcode.
A 6.6 kW system:
- Installed in 2026 (5 years): 6.6 x 1.382 x 5 = 45.6, rounded down to 45 STCs
- Installed in 2027 (4 years): 6.6 x 1.382 x 4 = 36.5, rounded down to 36 STCs
That is nine fewer certificates. At an illustrative $38 per STC, the discount falls by around $340 on the same system. For the customer, that is a price step on 1 January with no change to the product.
Check the current price on the pricing page and run the same sum for your most common system sizes.
Why December installs matter
The deeming period depends on installation date, so a job physically completed in December 2026 earns the five-year figure. A job completed on 2 January 2027 earns four.
That creates two pressures:
- Customers who delay. Anyone deciding in October or November may be motivated to proceed before the year ends. You can tell them the truth: the discount drops at the start of January.
- Your own workload. December is busy. Weather, parts supply and holiday shutdowns can push jobs across the line.
Lodge December installs early
For jobs genuinely installed before the cut-over, lodge them promptly. This does not change the deeming period, which is fixed by installation date, but it reduces risk:
- Photos and details are fresh.
- Queries can be resolved before staff go on leave.
- You are not chasing signatures in January.
- Your cash comes in before the holiday shutdown.
Our desk works through the usual holiday period with published hours, so tell your account manager what is coming. If you expect a surge, a heads-up helps us plan. Settlement runs within 24 hours of sign-off; see how it works.
Planning quotes across the year boundary
Practical steps for the next few months:
1. Show the date dependency
If you quote in November for an install in January, the quote should state that the STC discount depends on the installation date. Otherwise you face a margin hit or an awkward conversation.
2. Build a December schedule
List signed jobs and work out which can be completed before 1 January. Prioritise by value and by the risk of slipping.
3. Re-price January quotes
Refresh your price list for the 4-year period. A system whose price looks competitive in December may need a different structure in January.
4. Avoid shortcuts
Do not backdate paperwork or claim an installation date that does not match reality. Audit findings can be costly. See how STC audits work.
5. Review your trader terms
With shrinking certificate counts, the rate and payment speed matter more per job, not less. Look at net dollars and timing, as set out in what an STC is worth in 2026.
What about batteries
Batteries work differently. Their discount depends on usable capacity and a factor that also steps down annually to 2030. See the installer’s guide to the Cheaper Home Batteries Program and our battery STCs page.
The longer view
The scheme winds down on a published timetable. That is a certainty most markets would envy. Installers who plan around it, quoting with the date in mind and keeping paperwork clean, will do better than those surprised each January.
For your solar certificate flow, the STC trading page explains how we work, and you can start trading before the year-end rush.