The STC calculation explained: what your solar discount is really worth in 2026
Every solar quote in Australia includes a discount from Small-scale Technology Certificates, or STCs. Almost nobody explains how it is calculated, and much of what you read online uses numbers from years ago. Here is the actual maths, current as of 2026.
The formula
STCs earned = system size (kW) x your zone rating x the deeming period (years)
Three inputs:
- System size: the panel capacity in kilowatts, for example 6.6.
- Zone rating: Australia is split into four solar zones by sunshine. Melbourne sits in Zone 4, the lowest, with a rating of 1.185. Brisbane and most of Queensland sit in sunnier zones with higher ratings, which is why interstate quotes look more generous.
- Deeming period: the years remaining until the scheme ends on 31 December 2030. Install in 2026 and you get 5 years. In 2027 it drops to 4, then 3, then 2, then 1, then nothing.
Worked example: 6.6kW in Melbourne, installed in 2026
6.6 x 1.185 x 5 = 39.1, rounded down to 39 STCs.
Each STC trades at roughly $38 to $40. So:
39 x $38 to $40 = roughly $1,480 to $1,560 off your system price.
Your installer claims the certificates and applies the discount at the point of sale, which is why you never see STCs as a separate step.
Why you read much bigger numbers online
Search for "STC rebate 6.6kW" and you will find claims of $2,000, $3,000, even 88 certificates. Those numbers were once true. When the deeming period was 11 years, the same Melbourne system earned 86 certificates worth over $3,000. Every January the deeming period shrinks, the certificate count drops, and half the internet forgets to update. It is a useful test of any solar website's freshness, including ours: we last checked these figures on 31 August 2026.
What this means for timing
The discount falls by roughly $300 each new year for a 6.6kW Melbourne system (one deeming year, about 8 certificates, at about $39 each). That is real money but rarely a reason to rush a decision by itself: a well-chosen installer and system is worth more than a December deadline sprint. It does mean "the rebate is ending, sign today" pressure tactics contain a grain of truth wrapped in a lot of exaggeration: the scheme steps down annually and ends after 2030, but it does not vanish overnight.
Checking your own numbers
The Clean Energy Regulator publishes the zone map and rules, and any itemised quote should show the STC discount as its own line. If a quote's STC figure looks far above the maths here, the installer is either using stale numbers or padding: either way, ask.
Next in this series: our first panel research report, on the most-mentioned panel brand across Melbourne installer websites.