Follow the sun: Big batteries now key players in evening peaks, and as clouds smother PV output

Australia’s growing fleet of big battery installations are taking an ever more prominent role in helping manage the variations in the output of wind and solar, as their primary focus switches from grid services to energy arbitrage.

The first big batteries on the Australian grid – from the Hornsdale battery on – were primarily focused on the sort of services that are barely visible, but also critically important, providing services such as frequency control and network support, particularly when sudden events – such as storms or generator trips – threaten grid stability.

But as batteries become bigger, and with longer storage durations, their influence on the balance of supply is becoming more obvious.

In the last 10 days this has been apparent in several ways. Firstly is the growing share of supplying the evening peaks. In South Australia, that rose to 17 per cent last Tuesday as batteries filled the gap created by the rapidly depleting output of rooftop solar in the evening demand peak.

Nearly every evening last week, batteries provided more than 10 per cent of local demand in the peak period, although the record share is 20.8 per cent – reached in September 2023, according to GPE NEMLog. But such excursions are common (see graph above).

This is now being replicated in the main Western Australia grid, where battery storage – likely for the first time – exceeded more than 10 per cent of the evening peak demand last Wednesday, again as the solar output faded with the setting sun.

Both markets are significant because they are global leaders for varying reasons. Both have significant amounts of rooftop solar – in South Australia, rooftop PV output can on occasions exceed 100 per cent of local demand. And the average share of wind and solar in the grid is around 75 per cent over the past 12 months.

Western Australia has a lower overall share of wind and solar (40 per cent), but it is also an isolated grid – the world’s biggest – with no connections to other states or countries. And it also has a high share of rooftop solar – which because of its isolation – presents greater challenges to the grid operator.

Because there is no option to export excess solar to another state, policy makers have been focused in recent years on a rapid deployment of big batteries, with half a dozen under construction, including two of the biggest in the country, and another four big batteries to be built within two years after winning a major tender last week.

Most of these battery projects have a particular focus – soaking up rooftop solar during the middle of the day, and injecting it back into the grid in the evening peak. This will be critically important as the last of the state’s coal fired generators exit the grid as planned by the end of the decade.

There will be some 2 GW and nearly 8 GWh of this battery capacity by the end of 2027 – a significant amount in a grid with an average demand of just over 2 GW, and peak demand around 4.2 GW.

Its flexibility is already on show. An event chronicled by the Australian Energy Market Operator on March 13 shows the impact of a passing thunderstorm on the output of rooftop PV in the W.A. grid.

According to AEMO, the thunderstorm and cloud cover caused a sudden drop in generation from rooftop solar, resulting in a rapid upswing in electricity from the grid, with demand surging 49 per cent from 1,600 MW to 2,385 MW in 40 minutes.

What is interesting to note is the reaction of the big batteries – for the moment limited to the two major operating units at Collie and Kwinana. Their remit is to charge during the four hour midday peak, but you can see in the graph above that as the clouds passed, and the output of rooftop PV plunged, the batteries quickly stopped charging.

That reduced the impact on the rest of the grid – less charging means lower demand – and the batteries even briefly injected some of that power back into the grid. As the clouds passed later in the day, the charging resumed before the batteries injected power back into the grid again in the evening peak.

The existing Kwinana and Collie batteries only have operating capacity of around 320 MW, so by the time that the committed capacity of 2 GW is in place by the end of 2027, they will be in a position to fully account for any variations in output caused by passing clouds and thunderstorms.

See Renew Economy’s Big Battery Storage Map for more details.