Customers add panels all the time, after a new roof, a bigger family or a new battery. The STC answer depends on classification.
How STCs work for additions
For an addition, STCs are created for the extra capacity, using the zone for the site and the deeming period for the year of the addition. For 2026 that is five years, falling to four in 2027.
Example: adding 3.3 kW in zone 3 in 2026 gives 3.3 x 1.382 x 5, which is about 22 STCs. The same job in 2027 gives 3.3 x 1.382 x 4, about 18.
The existing panels earned or did not earn their STCs when they were installed. They do not earn again.
Conditions to check
- Classify it as an addition, not a new system or a replacement. The installation type guide explains each.
- Added panels must be new and on the approved list, with accredited installation.
- The registry record for the existing system needs to be consistent with the claimed capacity, or the claim may be queried.
- Total system size matters for the small-scale limit, which is 100 kW (above that, up to 1 MW, the mid-scale rules apply to installs from 1 October 2026).
- Evidence. Photos of the new panels, serials and the arrangement with the existing array.
Replacements and upgrades
If you swap old panels for larger ones, the claim covers only the net increase, and you may need to evidence the removal. This is where mistakes happen, so photograph the old array before you remove it.
A worked example
A customer has a 5 kW system installed in 2019 and wants 3 kW more in October 2026 in zone 3. The 5 kW system earned STCs when it was installed, so those are not repeated. The new 3 kW creates 3 x 1.382 x 5, about 20 STCs. The combined system is 8 kW, well below the 100 kW line. The inverter may need upgrading, and that is a separate cost with no STC, so say so on the quote.
What this means for installers
Write the classification into the job sheet. Many “failed” claims come from an addition lodged as a new system. See how to fix a failed STC claim, the photo requirements and the STC trading page. Our pricing page shows the current rate.