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Until the start of May, the federal battery rebate paid the same amount for every kilowatt hour you installed. The 30th kilowatt hour earned what the first one did. So the sensible move was to go big, and a lot of people did.
That stopped on 1 May 2026. The rebate now tapers:
Battery STC factor · From 1 May 2026
More capacity.
A smaller rate on the extra.
First 14 kWh
Next 14 kWh
Next 22 kWh
Width shows capacity. Height shows the share of the STC factor.
41 kWh, split across the bands.
| Usable capacity | STC factor applied |
|---|---|
| 14 kWh | 100% |
| 14 kWh | 60% |
| 13 kWh | 15% |
These percentages apply to the STC factor for each band, not to the purchase price or the whole battery. Eligibility, installation date and certificate value also affect the discount.
Source: DCCEEW · battery STCs ↗
New national figures out this week show how fast people worked that out. In April, batteries of about 41kWh were roughly a third of everything installed in Australia. By July, no single size was above about 5 per cent. One month of a changed rebate scattered the market.
41kWh made sense in April and does not now
Run the numbers on a 41kWh battery under the new bands. The first 14kWh gets the full rate. The next 14kWh gets 60 per cent. The last 13kWh gets 15 per cent.
That last chunk is a third of the battery you are paying for and it brings back almost none of the discount. It is the most expensive capacity on the wall.
Under the old rules it was the cheapest. Nothing about the battery changed. The rebate did.
Sizing for evening consumption
Here is the part that costs us money to say: most Perth homes should be buying a smaller battery than they would have been quoted in April, and we would rather tell you that than sell you 40kWh you will never cycle.
A three or four bedroom house in Perth, single phase, ducted aircon running into the evening, typically lands somewhere in the 13 to 20kWh range for the battery to earn its keep. That sits almost entirely inside the two better rebate bands. Push past 28kWh and you are buying capacity at close to retail while your neighbour's smaller system got the discount.
Bigger still makes sense for some houses. A five bedroom on three phase with a pool, a spa and two EVs charging overnight will use it. A workshop out the back with a welder in it, same. If that is your house, buy the capacity, just go in knowing the top of it is near full price now.
The number that decides this is your evening and overnight consumption, not the size of your roof and not the size of your solar array. Pull twelve months of Synergy data and look at what you draw after sunset. That is the battery you need.
Check discounts on quotes written before May
If you are holding a quote from March or April, or one that has been sitting open since then, check what rebate figure it is built on. The discount depends on the date the battery is installed, not the date the quote was written. A quote priced on the old flat rate and installed now does not deliver the number on the page.
Anyone who is mid-way through comparing quotes right now is comparing them across a rule change, which is a bad time to assume two numbers were worked out the same way.
If you want the certificate arithmetic worked through band by band, with the numbers, we did that at length on PSW Energy: the third 14kWh of battery earns about a seventh of the first.
If you want a size worked out off your own consumption rather than off a package, get a quote and bring twelve months of bills.