The 2026 STC step-down, in plain numbers
Deeming falls to five years for anything installed in 2026, and it falls again every January until the scheme closes on 31 December 2030. Here is what that actually does to a 6.6 kW quote — with the arithmetic on the page rather than described.
The figures below are the ones our STC engine applies. AU · CER
Every quarter someone asks why the same system is worth less this year than last, and the answer is always the same two words: deeming period. It is not a price change and it is not a policy anyone announced recently. It was written into the scheme when it was designed, and it steps down on 1 January, every year, until there is nothing left to step down.
What deeming actually is
A small-scale system does not earn certificates for the power it generates. It earns them up front, for the power it is expected to generate between installation and the end of the scheme. That expected run is the deeming period. In 2026 it is five years, because 2026 to 2030 is five years. In 2027 it will be four, for exactly the same reason.
So the certificate count for a system is, roughly:
A 6.6 kW system in Zone 3, year by year
Zone 3 covers Sydney, Adelaide and Perth, at a rating of 1.382. Melbourne is Zone 4 at 1.185 — the step-down is identical, it just works from a smaller base. At an indicative $38 per certificate:
| Install year | Deeming | Certificates | At $38 | Change |
|---|---|---|---|---|
| 2025 | 6 years | 54 | $2,052 | — |
| 2026 | 5 years | 45 | $1,710 | −$342 |
| 2027 | 4 years | 36 | $1,368 | −$342 |
| 2028 | 3 years | 27 | $1,026 | −$342 |
The step is about $342 a year on a 6.6 kW system in Zone 3 — nine certificates, every January. The same system in Melbourne loses seven a year rather than nine, because the zone rating is lower, not because the rule is different. On a 13 kW commercial job it is roughly double either figure.
Why it bites in December and not January
The certificate count is set by the installation date, not the date the quote was written or the claim was lodged. A job quoted in November for a January install is quoted at this year's deeming and installed at next year's — and the difference comes out of somebody's margin. Usually the installer's, because the customer has a signed number.
The fix is unglamorous: if a December quote might slip, price it at the deeming it will actually be installed under, and say so on the document. A quote that explains why the rebate figure changes after 31 December is a quote that does not need renegotiating in February.
What we do about it
SunLedger computes the certificate count from the install date on the job, not from today — so moving an install date across 1 January updates the rebate, the net cost and the payback on the quote, before it goes out. The zone comes from the postcode, and the deeming from the calendar. Neither is typed in, so neither can be stale.