Marketing sheets quote nominal capacity, because it is the bigger number. The certificate maths uses a different one.
Nominal versus usable
Nominal capacity is the total energy the battery is rated to hold. Usable capacity is the portion you can actually draw out, after the battery’s depth-of-discharge limit and other reserves. A 10 kWh nominal battery with a 90% depth of discharge has 9 kWh usable.
Which one counts for STCs
The certificate formula runs on usable capacity:
STCs = usable kWh (through the tiers) x the factor for the commissioning period, rounded down.
The tiers (first 14 kWh at 100%, 14 to 28 kWh at 60%, 28 to 50 kWh at 15%) and the 50 kWh cap on certificates are also applied to usable capacity.
The program’s eligibility range, a battery system of 5 kWh to 100 kWh, is set on nominal capacity. So nominal decides whether the system is in the program, and usable decides how many certificates it earns. The example from the regulator: a 14 kWh battery commissioned when the factor is 6.8 creates 14 x 6.8 = 95.2, which rounds down to 95 STCs.
How usable capacity is determined
The figure is taken from the Clean Energy Council approved battery list, which records the capacity for each approved model, and the installer lodges it with the claim. The installer does not choose it, and a customer’s own measurement of how much the battery delivers does not replace it. If a manufacturer lists several configurations, use the one that matches what was installed. See the CEC list.
The 5 kWh minimum rule
A battery system under 5 kWh nominal is not eligible. The edge case is a unit near 5 kWh nominal, whose usable figure is less than 5. The size range is set on nominal capacity, so such a unit can qualify while earning fewer than 34 STCs at the 6.8 factor. If you are close to the line, confirm with the regulator before the customer is quoted. For expansions, the added capacity must be at least 5 kWh nominal; see expanding an existing battery.
A worked example
A battery labelled 13.5 kWh with 13.5 kWh usable earns 91 STCs at 6.8. A battery labelled 15 kWh with 13.5 kWh usable earns the same 91, because the usable figure is the same. A 15 kWh usable battery earns 99, since the extra kilowatt-hour is counted at 60%. Check any size in the battery STC calculator.
What this means for installers
Build a habit of pulling the usable figure from the CEC list at quote stage and writing it on the quote. Mismatches between the figure used and the listing are an easy rejection. Our compliance desk checks this before lodgement; see /start-trading/ and /pricing/. The battery submission guide lists the evidence required.
What this means for you
Ask for usable kWh on the quote and the STC count that follows from it. The battery STCs pillar covers the program, and why the rebate is smaller above 14 kWh explains the tiers.