What is renewable energy curtailment and how does it affect rooftop solar?

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.

A bar graph showing rates of curtailment rising to 2050
An influx of renewables will sharply increase the rate of curtailed energy.(Supplied: AEMO)

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.

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