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Buy the battery that matches what you use after sunset. Since 1 May, capacity beyond that is close to unsubsidised, and the arithmetic is no longer subtle.
Here is the change. The certificate factor dropped from 8.4 to 6.8, and support became tiered by capacity: the first 14kWh attracts the full factor, capacity between 14kWh and 28kWh attracts 60 per cent of it, and capacity between 28kWh and 50kWh attracts 15 per cent.
Run that out on one system and the shape of it becomes obvious.
| Battery size | Approximate certificates | What the extra 14kWh added |
|---|---|---|
| 14kWh | 95 | n/a |
| 28kWh | 152 | 57 |
| 42kWh | 166 | 14 |
Those totals are indicative. The entitlement is calculated across the whole system and rounded down, so treat them as the shape of the curve rather than a quote.
The third block of 14kWh earns about 14 certificates where the first earned about 95. Going from 28kWh to 42kWh is 50 per cent more battery for roughly 9 per cent more certificates. You are paying close to full retail for that capacity, and you are paying it up front on a product that will spend most of its life sitting at a state of charge it never needed.
Battery registration trends
Average registered system size nationally fell from 44kWh in April to 38kWh in May and 26.16kWh in June. Registered capacity fell alongside it, from 2.43GWh in April to 1.51GWh in May and 1.07GWh in June.
Most of the coverage read that as a collapse in demand. It is not, for two reasons.
The first is that April was inflated. Everyone who could bring a purchase forward ahead of 1 May did, which is what a deadline does. Comparing anything to April is comparing it to a pulled-forward peak.
The second is administrative, and almost nobody accounts for it. Certificates can be created at any point up to 12 months after the installation date, which means the most recent month in any published series is always counted short and fills in later as the paperwork arrives. June's figure will rise. It has not finished being June yet.
Strip both of those out and what is left is buyers re-sizing. Average capacity landed at 26.16kWh, which sits just inside the 14 to 28kWh band where the tier still returns 60 per cent. That is not a coincidence and it is not caution. It is people finding the edge of the incentive and stopping there. SunWiz, whose data the monthly series comes from, calls it a controlled reset rather than a structural decline. That reads about right from where we sit.
Rooftop solar has not slowed at all. It is running roughly 41 per cent ahead of the same point last year, which is worth holding in mind if anyone tells you the whole sector has gone quiet.
Size storage to evening demand
We would rather sell you a 28kWh battery than a 42kWh one. If 14kWh covers your evening, we would rather sell you that. Oversized storage is the most common way we see people waste money on an otherwise sound system, and the tiers have turned a mild inefficiency into a costly one.
The test is your overnight and evening load, not your daily total and not your system size. A three-bedroom home without ducted air conditioning frequently draws less between sunset and sunrise than the brochure numbers imply, and a 14kWh battery covers it with margin. A five-bedroom home running ducted air conditioning past 9pm is a different profile entirely, and there the second tier earns its keep.
The cases where we will point you above 28kWh are narrow and specific. Genuine off-grid or heavily backed-up sites. A property with an unreliable supply where whole-of-house backup through a long outage is the actual requirement. A business with a real evening or overnight load profile. If you are in one of those, the 15 per cent tier is a poor return but the capacity is still the right call, and we will say so.
If you are not, the money is better spent on the parts of the system that do not degrade. More panels. A better inverter. A properly engineered switchboard and a backup circuit arrangement that works when you need it. Who installs it matters more than which battery is on the wall, and that has not changed with the tiers.
Check the certificate calculation
Ask any quote you receive to show the certificate contribution at the tiered rate rather than the headline factor. A quote that applies 6.8 across the whole capacity overstates the discount on anything above 14kWh, and it will overstate it substantially above 28kWh. That is an easy error to make and a costly one to accept.
The 1 May settings and the 12-month certificate window are both published by the Clean Energy Regulator. The monthly capacity and system size figures are SunWiz's. If you want the numbers in front of you before you talk to anyone, they are worth 10 minutes.
Get a quote, or send us your last 12 months of consumption data and we will size it to the load rather than to the brochure.