The STC Effect Explained

The battery market didn’t crash in May. It just filed its paperwork late.

In summary

• The “STC effect” describes the distortion created when the reported market, based on Small-scale Technology Certificate creation, lags the real market, based on installation, making sharp movements look sharper than they are.
•  A record-breaking April was always going to give way to a quieter May and June, as the demand the rebate deadline had pulled forward gradually unwound, which is the solar-coaster behaving exactly as it always does.
• Those headline monthly numbers overstate the drop, because battery registrations can lag installation by as much as twelve months and leave recent months undercounted until the paperwork catches up. Reading the market by installation date gives a truer and steadier picture.

There is a discomfort in watching your own industry lead the evening news with headlines that, on the surface, look like something people should be concerned about. Last month, it was the home battery market’s turn, after the national broadcaster reported that installations had dropped sharply once the government trimmed its rebate. The media and stakeholder calls that followed came, as they tend to when this market lurches, to SunWiz, and what we told reporters was more encouraging than the figure beneath the headlines.

Almost nobody who follows this market was caught out by the fall. Incentives with a deadline behave predictably: demand piles up before the cutoff and empties out after it, and the steeper the run-up, the sharper the lull. April produced a record because everyone holding a quote rushed to beat the change, and May and June handed much of that momentum back. I have taken to calling it a controlled reset, choosing the word controlled with care, because this is the solar-coaster doing what it always does when the rules are rewritten.

What has been missing from the discussion is critical: we call it the STC Effect and once you understand it, the data and the future market direction becomes a lot clearer. This is where installation-level data helps you see the true picture while everyone else is transfixed by the dip. Here it is:

Battery rebates here are claimed through Small-scale Technology Certificates, and an installer has up to twelve months to create those certificates once a system is on the roof. Any recent month is incomplete when first reported, filling in as the paperwork catches up, so a steep early print for May or June is all but guaranteed to soften as the late registrations land. The regulator’s monthly figure is better read as an early draft than a final account, and anyone treating it as final will watch an orderly cooldown look like a collapse. 

SunWiz corrects for the STC Effect by tracking the market by the month a battery is installed rather than the month its certificate is lodged, letting us estimate where a month will settle long before the official record catches up.

Once the headline is set aside, the picture turns from worrying to reassuring. The change that took effect on 1 May was built to rein in the oversized forty and fifty kilowatt-hour systems that had been absorbing the largest share of the subsidy, and it is those outsized installations, rather than demand in general, that have pulled back. In their place has come a swell of mid-scale systems in the twenty to thirty kilowatt-hour range, now the market’s centre of gravity, which is to say households are buying storage better matched to how they live. A market that sheds its excesses and settles on sensible sizing looks more like an industry growing up than one in retreat.

It lays out the same market on three clocks: the view drawn from certificate creation, the view drawn from installation dates, and our estimate of where the numbers settle once every certificate has been counted. What the three share is their shape, that of a reset landing on firmer ground rather than a market sliding off a table, and the gap between the raw print and the firmed read is the margin that tips an industry into a panic it does not need.

None of this is to pretend a cooling market cannot be read as bad news, and plenty will read it that way, but I would push back on the instinct. A market that can reach a record in a single month is hardly starved of demand, and most of the sales that vanished in May and June were ones April had already borrowed rather than ones that ceased to exist. Some of those April mega-systems were doubtless larger than their owners needed, which is why tapering the subsidy made sense, but reining in an over-generous incentive is a world apart from households losing their appetite for storage. Last year the battery market roughly tripled to 221,000 systems and moved from niche to mainstream, and rooftop solar is enjoying its strongest first half on record, around 41% ahead of this time last year.

If there is a lesson here, it is that the solar-coaster is in no hurry to pull into the station. The program’s subsidies step down every six months until they expire in 2030, and each reduction will bring its own rush and lull, its own alarming print and quiet recovery. The operators who prosper will be those who read the trajectory beneath the noise rather than flinch at every headline, and there is plenty to feel good about: SunWiz expects around 350,000 installations across 2026, and the market beneath the turbulence is larger and better proportioned than a year ago.