Owners often outgrow their first system, or decide to add a separate one for a pool shed, workshop or granny flat. The scheme anticipates this with a specific installation type: the additional system.
What counts as an additional system
The CER defines an additional system as a new, separate system at the same address. It has its own inverter and its own panels. The test for STCs is that the new system is built from new approved components, installed by an accredited installer, and that the combined capacity of all small-scale systems at that address is under 100 kW. Above that, the arrangement may need to be assessed as a power station, or fall under the new mid-scale solar STC rules if installed from 1 October 2026.
Same property, same roof
There is no rule against two systems sharing a roof. A second system on the same roof is as eligible as one on a separate building, provided the installation is safe and compliant. In practice, shading, roof space and the network connection limit more often than the STC rules do. Many networks cap exports or total inverter capacity per connection, so check with the distributor.
Worked example
A home has a 6.6 kW system from 2019. In 2026 the owner adds a separate 5 kW array on a new inverter, in zone 3. The new system earns 5 x 1.382 x 5 = 34 STCs. At the roughly $38 to $40 STC spot range at the time of writing, that is about $1,300 to $1,400. The first system earns nothing new. If the same panels were connected to the old inverter instead, the job would be an extension; see adding panels to an existing system.
Second systems for granny flats and outbuildings
A secondary dwelling with its own system is a common additional system. See granny flat solar STCs. The same reasoning applies to a workshop or shed: the question is whether the system is a genuine installation with a load, not what the building is called. See shed solar STCs.
What this means for installers
Choose the installation type and be consistent: the installation type guide explains the options. Add up the capacity of every system at the address before you quote, including any old systems not installed by you. Record the old system’s size on the file. Take photos that show both systems and their separation.
Once lodged, see STC trading and current rates, or start trading.