Some Australian states are discovering what happens when they have too much rooftop solar

As Victoria enjoys mostly sunny spring weather this weekend, energy authorities will be monitoring the extent to which electricity demand falls. If necessary, they switch off the solar systems on the roofs to ensure the stability of the electricity grid.

Such minimum system load events, as they are called, have become a new challenge as households across Australia take advantage of falling solar panel prices to protect themselves from rising energy bills and reduce carbon emissions.

Australia’s energy market operator issued two such warnings for Victoria during the AFL grand final weekend a fortnight ago, and warned of two more for next Saturday and Sunday.

Prior to this cluster, the state’s only previous warning was last December 31.

The public has become accustomed to annual warnings about possible power shortages during summer heat waves or prolonged cold spells in winter.

It won’t be long before the reverse – a grid strained by too little demand – will be common during mild, sunny spring and autumn days, when the need for cooling or heating decreases, experts say.

Related: Amid Australia’s chaotic climate politics, the rise of rooftop solar is an unlikely triumph | Adam Morton

“It’s all getting uncomfortably interesting for energy system people,” says Tennant Reed, director of climate change and energy at Ai Group, noting that “regulations are emerging to keep the show going.”

Having a grid powered entirely by renewable energy is something that Aemo and the state and federal governments have anticipated as they ramped up support for so-called consumer energy sources. Australian households have already embraced rooftop solar at a rate unmatched anywhere else, with more than a third generating electricity at home.

There are many options available to meet the extra demand, such as encouraging people to use more of their energy during sunny spells, renewable energy advocates say.

Hot water heaters, which now often operate at night, could be turned on during the day, while certain industrial users could be offered incentives to increase production, just as they are now rewarded for turning off power during summer heat waves.

Yet the looming challenges are not small, especially now that coal-fired power stations are closing.

The system strength of the electricity grid is “expected to decline sharply over the next decade,” Aemo said in its latest 2024 Electricity Statement of Opportunities report.

From October to December, demand is likely to be at its lowest for most parts of the national electricity market. (The National Electricity Market or Nem serves all regions except Western Australia and the Northern Territory.)

Windy, sunny spring days – much like the coming weekend in Victoria – also mean an increased supply of renewable energy in a season with minimal heating or cooling needs.

A year ago, on October 29, demand on the Nem network reached a record minimum of about 11 gigawatts for 30 minutes. Small-scale solar energy, most of it on residential roofs, met 52% of underlying demand.

As more homes install solar, Nem’s minimum demand could continue to shrink at its current rate of 1.2 GW per year, Aemo said.

The Nem needs at least 4.3 GW of electricity moving across its transmission network. If there are “unplanned network or unit outages,” the threshold rises to around 7 GW – a level that could be exceeded as soon as next spring, Aemo said.

“While these periods of very high distributed PV levels relative to underlying demand are currently infrequent, they will increase over time and may occur during unusual events and outage conditions,” the report said.

“A credible disruption could lead to a reliance on emergency frequency control systems that are known to be compromised during periods of such low operational demand, escalating the risks of system collapse and power outages.”

However, the market has an extensive toolkit at its disposal to address these risks. South Australia, where around half of homes have solar panels, has already experienced two Level 3 minimum system load events – the most severe – on October 11, 2020 and March 14, 2021.

For the latter, SA Power Networks, a company partly owned by Hong Kong-based conglomerate Cheung Kong Infrastructure, was ordered to switch off 71 megawatts of photovoltaic systems. It was Aemo’s first such intervention.

Related: South Australia aims for 100% renewable energy by 2027. It is already ‘remarkable’ internationally

So-called solar curtailment is now a feature of SA’s operating system, even if such intervention is intended as a “last resort”, the networking group said.

Before such action, large-scale solar and wind farms will be turned off, as well as large solar energy systems on shopping centers and factories. The export of excess solar energy from homes will also be stopped before the systems themselves are switched off.

According to WattClarity, a leading energy data website, South Africa had at least eight such rooftop solar restrictions in 2022 and 2023.

Victoria, which introduced similar ‘backstop’ rules on October 1 this year, says consumers can do their bit. Restricting solar energy should be the authorities’ last step.

“We encourage households with solar panels to make the most of their clean energy and save on bills by using their solar energy during the day – whether it’s charging electric vehicles, doing laundry or running the dishwasher,” a Victorian government spokesperson said.

For Victoria, the backstop mechanism will not impact a household’s ability to use its own solar energy. Large batteries will be part of the toolkit from this spring, with storage on standby to create additional demand by charging.

New South Wales and Queensland are expected to face similar challenges in the coming years. However, NSW has yet to begin a public consultation on restrictions on solar exports, with minimum load issues not expected until late 2025 or beyond.

Wayne Smith, acting CEO of the Smart Energy Council, said the best way to solve the problem would be to encourage more adoption of home batteries, not restrict rooftop solar. (About one in five new solar systems contains a battery, says consulting firm SunWiz.)

“The abundance of rooftop solar is a gift from Australian households who have spent their own money to produce energy from their own homes,” Smith said.

“Too often, governments and regulators view solar energy as a problem to be managed, rather than as an opportunity to be seized.”