When you install rooftop solar, you do not wait years for a rebate to trickle in. The incentive is paid once, at the start, as a bundle of certificates. The deeming period is how many years of generation that bundle covers.
How deeming works
The scheme does not measure what your panels produce. It estimates it. The CER takes the system’s capacity in kW, the zone rating for the postcode (a proxy for sunshine), and the deeming period in years. Multiply the three and you get megawatt hours “deemed” to be generated, and each one becomes a certificate: kW x zone rating x years, rounded down. See the STC formula.
So for a 6.6 kW system in zone 3 (rating 1.382), in 2026 (5 years): 6.6 x 1.382 x 5 = 45.6, or 45 STCs.
What it is not
It is not a warranty period, not the life of the system, and not a time limit on the savings. The panels keep producing for decades. The deeming period only decides how much of that future output is credited at once. Because it is credited at the start, a household can take the value as a discount on day one instead of waiting.
2026 and beyond
| Install year | Deeming period |
|---|---|
| 2026 | 5 years |
| 2027 | 4 years |
| 2028 | 3 years |
| 2029 | 2 years |
| 2030 | 1 year |
The scheme ends on 31 December 2030. See solar deeming period by year for what that does to the STC count on a 6.6 kW system, and why it falls each year for the reasoning.
What this means for installers
The deeming period is why quotes lose value each January. Build it into your sales calendar: the install date sets the period, so a job commissioned on 31 December earns a fifth more STCs than one commissioned the next day. Ask your customers to plan around the date, not the contract. Read the deeming period and zone ratings resource, the deeming period pillar, and our January 2027 explainer.
Check any quote with the STC calculator and compare rates on pricing. More definitions are in the resources library and the glossary entry.