Two Powerwall-sized units is a popular size for large homes. The question is whether you get the rebate twice. You do not, and the tiers are the reason.
The calculation for 27 kWh
Using the 2026 factor of 6.8 and the tiers in force since 1 May 2026:
| Slice of capacity | kWh | Share of factor | STCs |
|---|---|---|---|
| First 14 kWh | 14 | 100% | 95.2 |
| 14 to 27 kWh | 13 | 60% | 53.0 |
| Total | 27 | 148.2, rounded down to 148 |
At an STC price of $38 to $40 (the range at the time of writing), 148 STCs is about $5,600 to $5,900. A single 13.5 kWh battery earns about 91 STCs, so the second unit adds only about 57. Check the actual figure for your usable capacity on the battery STC calculator.
The usable capacity of a 13.5 kWh nominal unit is slightly less than 13.5 in some products, so the real figure can be a little lower. The certificate calculation uses the usable figure; see nominal versus usable.
Why it is one system, not two claims
The program allows one battery installation per premises. A system installed together, in one commissioning, is a single installation even when it has several units or modules. The tiers are applied once to the total. That is also why the rebate for “two batteries on the same house” is not simply twice the single-battery number. The related question, rebating two batteries at the same house under the tiers, has the same answer.
When two installs do not work
If the second battery goes in months or years after the first, it is a separate installation at an already-discounted premises, and it will not earn STCs. The exception is capacity added to a battery that never received a discount; see expanding an existing battery. If you think you may want 27 kWh, install it as one job.
Is the second unit worth it?
The second 13.5 kWh unit adds about $2,200 of certificate value on a purchase of 13.5 kWh more hardware. That may still be good value for a large household or backup needs, but the rebate does not carry it the way it carried the first. See is a 30 kWh battery worth it.
What this means for installers
Lodge one claim for the system, with serial numbers and photos for every unit. List each unit and the total usable kWh used in the calculation. Our compliance desk pre-checks multi-unit systems before lodgement, and you can see how certificate rates and settlement work on /pricing/ and /start-trading/. The battery STCs pillar has the wider context.
What this means for you
Ask for the STC count on the quote and compare it to 148 for a 27 kWh system at the 6.8 factor. If the discount looks like twice the single-battery figure, ask why.