The formula behind a solar STC claim is short. What trips people up is the inputs.
The formula
Number of STCs = system size (kW) x zone rating x deeming period (years), rounded down.
Each unit of the result is one megawatt hour of electricity the system is deemed to generate over its deeming period. That is why the same system creates fewer STCs each year even though it still produces the same power: the scheme counts fewer future years.
The three inputs
- System size. The rated capacity of the panels in kW, not the inverter. A 6.6 kW array of 15 x 440 W panels is 6.6 kW.
- Zone rating. Set by postcode from the CER’s table: zone 1 is 1.622, zone 2 is 1.536, zone 3 is 1.382, zone 4 is 1.185. Sydney, Brisbane, Perth, Adelaide and Canberra are zone 3; Melbourne and Hobart are zone 4. See deeming period and zone ratings.
- Deeming period. 5 years for installs in 2026, 4 for 2027, 3 for 2028, 2 for 2029 and 1 for 2030. The scheme ends 31 December 2030.
A worked example
6.6 kW in Sydney (zone 3), installed in 2026: 6.6 x 1.382 x 5 = 45.606. Rounded down, 45 STCs. At roughly $38 to $40 at the time of writing, that is about $1,710 to $1,800.
The same system in Melbourne (zone 4): 6.6 x 1.185 x 5 = 39.1, so 39 STCs. Installed in January 2027 in Sydney: 6.6 x 1.382 x 4 = 36.5, so 36 STCs.
Common mistakes
- Using inverter size instead of panel kW.
- Using the date of the quote instead of the installation date. The deeming year follows the installation.
- Rounding per kW before multiplying.
- Applying a zone by suburb name rather than postcode.
What this means for installers
Put the formula on your quoting template and let the STC calculator check it. If the count on your claim does not match the CER’s, it will be flagged, which holds the claim up. Batteries use a different formula, with a battery factor and kWh tiers; see how the battery STC is calculated. Rates for turning STCs into cash are on pricing.
For year-by-year numbers see deeming period by year. More at the FAQ or start trading.