This page covers both the 30 kWh and the 40 kWh version of the question, since the logic is the same. The tiered structure means the answer is rarely the rebate.
What the rebate actually adds
At the 2026 factor of 6.8, and with the tiers in force since 1 May 2026:
| Size | STCs | Value at $38 to $40 | Added over a 14 kWh battery |
|---|---|---|---|
| 14 kWh | 95 | $3,610 to $3,800 | |
| 28 kWh | 152 | $5,776 to $6,080 | about $2,200 to $2,300 |
| 30 kWh | 154 | $5,852 to $6,160 | about $2,300 to $2,400 |
| 40 kWh | 164 | $6,232 to $6,560 | about $2,600 to $2,800 |
So 26 kWh more storage than a 14 kWh battery brings about $2,600 to $2,800 more in certificates. Compare that with the extra hardware and installation cost on your quote. If the 40 kWh quote is $10,000 or more above the 14 kWh quote, the rebate covers a quarter of the gap or less.
When more capacity does pay
The capacity can be worth buying for reasons that have nothing to do with the discount:
- Evening load. Households with electric hot water, heat pumps, pool equipment or an EV may use well over 14 kWh after sunset.
- Backup. Longer outage cover needs more storage, and in bushfire or storm country that can be a real consideration.
- Tariffs and VPPs. Time-of-use pricing and VPP payments reward battery capacity that can export at peak. See the VPP incentive stack.
- Solar size. A large solar array can fill a bigger battery most days. If the panels only produce 8 kWh of surplus, a 30 kWh battery will never fill.
When it does not
If your daily surplus and evening use are both under about 14 kWh, the extra capacity sits idle. You then pay for 16 kWh or more of hardware that earns a small rebate and does not change your bill. Check the actual data from your meter before deciding.
The effect of timing
The factor drops to 5.7 on 1 January 2027 and 5.2 on 1 July 2027, so a 30 kWh battery that earns about 154 STCs now earns about 129 in the first half of 2027. For larger sizes the loss is smaller in dollars than for the first 14 kWh, but it is a real change. See the 2027 factor.
What this means for installers
Present the incremental price and incremental STCs. Customers who buy a larger battery on an unfair promise of “more rebate” are the ones who complain later. For settlement of larger-battery claims, see /start-trading/ and /pricing/.
What this means for you
Work out your own numbers with the battery STC calculator, and read is a battery worth it and the battery STCs pillar. For the underlying tiers see why the rebate is smaller above 14 kWh.