As more households install rooftop solar and more renewable energy farms come online, a new problem is emerging.
It's called "curtailment", which is where an electricity generating system stops exporting to the grid or even temporarily shuts down, effectively wasting energy that could have been used.
This week, Western Australia followed South Australia in granting authorities the power to turn off household solar systems when the electricity network is under severe stress.
But curtailment isn't only an issue in WA and SA.
Rooftop solar owners in every state and territory may be surprised to hear their panels are often being curtailed.
For a few unlucky homes, curtailment may be reducing annual output by up to 20 per cent — and the owners may not even know.
So what causes curtailment, how can it be avoided, and will it affect the uptake of rooftop solar?
Why we're increasingly using less energy than we can produce
To understand what causes curtailment, (and why your rooftop solar is sometimes not generating energy), we need to go into some detail about a fairly dry topic: our system of electricity generation and transmission, which we call the grid.
Electricity generation can be curtailed for economic or grid-capacity reasons.
For instance, when prices go negative in the grid, the operators of large wind and solar farms will choose to stop exporting, to avoid an economic cost.
Or when supply is so high that the amount of electricity being generated threatens to overwhelm the grid's capacity, the market operator will block supply.
Pretty simple, right?
For instance, on September 19 last year, conditions of high supply but low demand (a weekend day of sunny but mild weather) saw energy prices go negative in the National Electricity Market (or the NEM, which is the interconnected grid for all of Australia except WA and the NT).
As a result, solar farms and wind generators chose to stop exporting, and more than 3,000,000kW of wind and solar was curtailed.
But the next day was a weekday, which saw increased demand, and higher prices.
This time, the energy generators were happy to sell as much energy as they could to the grid, and the Australian Energy Market Operator (AEMO) had to step in and block supply, to avoid overloading the system.
So what does this have to do with rooftop solar? As more is installed in Australia, demand for electricity goes down and supply goes up.
This means that large utility-scale generators (e.g. solar and wind farms) are increasingly being curtailed, or curtailing themselves.
According to AEMO, curtailment is going up, both as an absolute value and as a proportion of total generation.
By 2050, over 20 per cent of renewable energy will be curtailed, it predicts.
That's well over 50 trillion Wh, which, to give a sense of scale, is more renewable energy than Australia currently generates.
How does it affect rooftop solar?
So you may be thinking, over-supply sounds bad for big producers of solar, but why does it affect my rooftop system?
It affects your system in two ways.
The first is that authorities are increasingly treating rooftop solar as a kind of massive but dispersed solar farm, which means they're beginning to control whether your system exports to the grid.
In Western Australia (from this week) and South Australia (since last year), the market operator has been granted the power to remotely switch off rooftop solar panels when the grid is at risk of being overloaded.
With some exceptions, this prevents the household exporting solar energy, as well as using solar to power its appliances.
Instead, with the panels switched off, the household relies on mains power.
The second way over-supply affects rooftop solar is much more common, but hasn't had as much attention.
When the household's solar inverter — which acts as an intermediary between the electricity grid and the home — senses too much voltage in the local grid, it automatically switches off the home's rooftop panels.
So, this form of curtailment is done by the inverter, rather than by the market operator.
This happens all the time and in every state and territory (not just SA and WA), says Baran Yildiz, a home energy expert at UNSW.
"This happens the majority of the day for some households," he said.
These figures come from a study conducted by Dr Yildiz and his UNSW colleagues, which analysed the household energy data from about 1,500 homes in Adelaide.
The results, published last year, give the first detailed picture of what has been a largely invisible problem: rooftop solar curtailment.
Most rooftop solar owners do not know when their rooftop system has temporarily turned itself off, or if they're one of the unlucky homes with up to 20 per cent curtailment, Dr Yildiz says.
"If they aren't monitoring and not looking at their apps ... it may take a while before they understand what's happening."
Curtailment has two economic impacts for households: less money from exporting electricity (admittedly, this usually isn't much) and having to pay for mains electricity.
"It may mean that instead of using solar electricity to run your appliances and air-conditioning, you may have to buy that electricity from the grid," Dr Yildiz said.
"At the moment the loss due to curtailment is quite small, but as we install more and more solar, that may increase."
'I think people were quite dismayed'
For some rooftop solar owners, curtailment boils down to a slightly lower return on their investment.
But due to the difficulty of measuring curtailment at the household level, few households are aware it's happening, says Sophie Adams, a co-author of the UNSW study.
"The issue is surprisingly invisible to networks and retailers — they don't know who it's actually affecting in the network," says Dr Adams, whose work looks at the social impacts of the transition to renewable energy.
As part of the study, researchers conducted interviews with South Australian households that have solar.
"I think people in general were quite dismayed to find that they were doing [curtailment]," she said.
"They'd chosen to put a solar system on the roof and most seemed to feel like they'd done that for a few reasons, including financial, but also thinking they were doing the right thing environmentally.
This wasn't the fault of the network or retailers, she added.
"They couldn't have been warned that this was happening, because it's such an emerging issue. Ten years ago, no-one could have imagined we'd be seeing this."
How can rooftop solar owners know if they're being curtailed?
The extent to which your rooftop solar system may be curtailed depends on several variables, including the local capacity or strength of the electricity network, and whether your neighbours also have rooftop solar.
"If every house has solar, then it's likely every house has curtailment," Dr Yildiz said.
Rural areas tend to have weaker networks, which makes them more prone to curtailment, he added.
"If you're in a rural area with a lot of solar, then you're likely to see curtailment."
More than a third of free-standing households in Australia have solar panels on their roof, with Queensland, South Australia and Western Australia close to 40 per cent, and some suburbs at over 50 per cent.
In the Perth suburb of Baldivis, for instance, more than 69 per cent of homes have solar.
To check if your system is being curtailed, you'll need a smart inverter that sends energy export data to a monitoring app.
Or just check your power bills for any big discrepancies.
What happens to the curtailed electricity?
It goes nowhere, because it doesn't exist.
That's because in order to produce electricity, the solar panel needs the potential difference provided by either the grid or household devices that consume electricity.
If we use the analogy of a water tank with a hole in the floor, the potential difference (another word for voltage) is the pressure acting on the water that's rushing out the bottom.
When the inverter disconnects the solar panel, that outlet is closed, so there's no potential difference.
So you don't have to worry about the unused panels electrifying your roof.
What can be done to reduce curtailment?
The two main solutions to curtailment are load-shifting and energy storage.
Load-shifting is about using energy-intensive appliances such as hot water systems and pool pumps in the middle of the day, to increase demand at that time.
By increasing demand, more energy is required to oversupply the grid, reducing the risk of curtailment (with the added benefit of lower demand in the evening, when there are fewer renewables).
Energy storage includes installing more batteries, either small ones serving individual homes, or larger ones for communities and entire grids.
Like load-shifting, energy storage increases demand when it's needed and stabilises sudden increases in grid-voltage.
These strategies can reduce curtailment, but some amount is inevitable.
In fact, curtailment can sometimes be more efficient than building more storage, according to the the AEMO draft 2022 Integrated System Plan:
"Adding more storage to soak up the surplus supply is unlikely to be economically efficient because, with so much annual renewable generation, there is little marginal value in shifting [variable renewable energy] to other times in the day, month or year ... installing sufficient VRE to meet the energy needs of winter and accepting some curtailment in summer is likely to be a more efficient outcome than the alternative of building less utility-scale solar but more seasonal storage."
So a little bit of wasted energy in summer is just the cost of having abundant and cheap renewable energy for the rest of the year.
Will this affect the uptake of rooftop solar?
For most homes, curtailment is currently a very small problem with minimal economic cost.
"The majority of households are not experiencing that much curtailment at all — maybe a couple of times a year," Dr Yildiz said.
That said, when they found out about curtailment, most households wanted to know more, and it made some "think twice" about rooftop solar, Dr Adams said.
"There was an expectation that they should be able to get advice about how much this would affect them," she said.
And there's always the chance you'll be the unlucky household with 20 per cent curtailment.
"Fairness is the important thing there," Dr Yildiz said.
"How do you find a way of fairly curtailing and distributing the cost of curtailment among a large number of users?"
