The quick formula is the same for every small-scale solar system. Once you know it, you can check any quote.
The formula
STCs = kW x zone rating x deeming period, rounded down.
For 2026 installs the deeming period is five years. The zone ratings are 1.622, 1.536, 1.382 and 1.185.
The 6.6 kW result by zone
- Zone 1: 6.6 x 1.622 x 5 = 53.5, so 53 STCs.
- Zone 2: 6.6 x 1.536 x 5 = 50.7, so 50 STCs.
- Zone 3: 6.6 x 1.382 x 5 = 45.6, so 45 STCs.
- Zone 4: 6.6 x 1.185 x 5 = 39.1, so 39 STCs.
What changes next year
From 1 January 2027, the deeming period falls to four years. Zone 3 becomes 6.6 x 1.382 x 4 = 36.5, so 36 STCs. The period then shortens by a year each January, to one year in 2030, and the scheme ends on 31 December 2030. The deeming period and zone ratings guide sets this out.
Common traps
- Using panel wattage, not system size. The capacity is the panel total, not the inverter size.
- Using the wrong zone. Look up the postcode, see zone rating by postcode.
- Using the wrong year. The install date decides the deeming period.
- Counting existing panels. For additions, only the new capacity counts.
Other common sizes
For a 5 kW system in zone 3 in 2026, the count is 5 x 1.382 x 5, about 34 STCs. For 8 kW it is about 55, for 10 kW about 69, and for 13 kW about 89. In 2027, with the deeming period at four years, those fall to about 27, 44, 55 and 71. A small table in your quoting tool saves arguments, and lets you show customers exactly what waiting until January costs them.
What this means for you
Homeowners can use the numbers to check a quote, and see the dollar value at STC value for a 6.6 kW system. Installers should build the formula into their quoting tools, and can see the current rate on pricing. The STC trading page explains how claims are sold.