Deeming period

Written and checked by the Energy Merchants desk · Reviewed 2 October 2026

The deeming period is the number of years of expected solar generation counted when a system’s STCs are created. It is set by the installation year, falls by one each year, and is one of the biggest drivers of how many certificates a system earns.

The scheme ends on 31 December 2030, so there is no deeming period beyond that. The schedule above is the one published at the time of writing.

Worked example

Take a 6.6 kW system in zone 3, with a zone rating of 1.382. A 2026 install earns 6.6 x 1.382 x 5 = 45.6, which rounds down to 45 STCs. The same system installed in 2027 earns 6.6 x 1.382 x 4 = 36.5, which rounds down to 36. At roughly $38 to $40 per STC, the nine certificates lost to the shorter period are worth around $340 to $360.

In practice

For an installer, the deeming period is a selling point and a deadline. Customers asked to sign in November have a clear reason not to wait until January. It also affects cash flow, because a mid-December install and a mid-January install of the same job can produce noticeably different payments. Plan the pipeline so jobs that are ready are installed and registered before the year changes. See what changes in January 2027.

Common confusion

The period depends on the installation date, not the contract date or the date the certificates are lodged. Use the correct installation date on the paperwork and keep photos that show it.

The deeming period also does not apply to batteries, which use usable capacity and a yearly STC factor instead. See STC factor.

For the full calculation, read deeming period and zone ratings and the certificate trading glossary.