Curtailment

Curtailment is the energy a wind or solar generator could have produced but wasn’t allowed or didn’t want to. It’s the difference between the resource a site has and the revenue a project can actually earn from it.

Two different things cause it, and Curtailment Total captures both:

• Network curtailment. A constraint is binding somewhere on the system, and the dispatch engine can’t accept everything the generator is offering without breaking a limit. The energy is available, the market wants it, the network can’t move it.
• Economic curtailment. The price has fallen below the level at which the generator is prepared to generate, often below zero, which happens in the middle of sunny, mild, low-demand days in high-penetration regions. The generator withdraws its own offer, or is priced out of the dispatch. The energy is available and the market has decided it isn’t worth taking.

The distinction matters commercially (one is a network problem, the other is a market-price problem), but they show up the same way in dispatch data, and RenewMap reports them together as a single figure. See Revenue & Missed Revenue to understand how the Missed Revenue metric can help you split out the difference.

What curtailment is not is a weather measure. A still, overcast day produces very little energy and zero curtailment, because nothing was available to curtail.

Why does curtailment only exist for wind and solar?

Curtailment is defined as generation that costs ‘nothing’ (zero marginal cost) which was not dispatched into the market. Technologies such as batteries, gas, coal or hydro have either an explicit cost (e.g. fuel) or implicit cost (opportunity cost of using energy now rather than saving it for later).

Wind and solar are different since they’re free if the resource is available. They get registered in the NEM as semi-scheduled units, and for those AEMO publishes a forecast of what each generator could have produced from the resource available to it, unconstrained by the network or the market. That forecast, published alongside the unit’s declared availability and its dispatch target, is what makes curtailment measurable at all.

So Curtailment Total is populated for semi-scheduled wind and solar, and blank for everything else.

Where you see it in RenewMap

How it’s calculated

Curtailment is available output less dispatched output, floored at zero, evaluated for every 5-minute dispatch interval and then averaged to the half hour.

For each interval, AEMO publishes what a semi-scheduled unit could have produced from the resource available to it, what it declared as available to the market, and the target output it was actually dispatched to. We take the more conservative of the two availability figures as the output genuinely available in that interval, and treat whatever share of it went undispatched as curtailment.

A generator can have a real resource but keep part of its capacity out of the market entirely (not bid at any price, for maintenance or otherwise).

Curtailment Percentage

Curtailment Percentage is curtailed energy as a share of available energy, meaning curtailed plus generated:

                       curtailed
Curtailment % = ------------------------- * 100
                 curtailed + generated

The denominator is deliberately not nameplate capacity. A still night with no output and no curtailment therefore doesn’t drag the percentage anywhere, and the figure stays comparable between a windy month and a calm one.

Where nothing was available at all (no generation and no curtailment, as on any solar generator at night), the percentage is left blank rather than reported as 0% or 100%.

Where the data comes from

The resource forecast, declared availability and dispatch target all come from AEMO’s published dispatch data. Whether a unit is semi-scheduled comes from AEMO’s registration data, resolved to the version in force at each interval, so a unit that changes classification is treated correctly on both sides of the change.

Common questions

Why is Generation populated but Curtailment is blank?

Generation only needs the unit’s own telemetry. Curtailment depends on the unit being involved in AEMO’s dispatch directions, which means it will only apply to semi-scheduled units (typically wind and solar >30MW).

Is a low output day the same as curtailment?

No. Curtailment compares available output against dispatched output. On a still or overcast day very little is available, so there’s very little to curtail: output is low and curtailment is near zero. A high-curtailment day is usually a good resource day where the network or the price got in the way.

Does this include curtailment the operator chose?

Yes. Economic curtailment (withdrawing capacity when prices are low or negative) is included alongside network curtailment. See Revenue & Missed Revenue to understand how the Missed Revenue metric can help you split out the difference.

Why doesn’t this match the developer’s own curtailment figure?

Project owners typically measure against their own resource assessment or their own estimate of potential output, not AEMO’s published forecast, and they often report only involuntary curtailment. Both differences push in the same direction as their reporting boundary. RenewMap’s figure is derived consistently from AEMO’s published data across every project, which is what makes it comparable between assets; it isn’t a substitute for an owner’s own energy accounting.

Related pages

• Generation & Load: what the unit actually produced
• Revenue & Missed Revenue: what the curtailed energy would have been worth
• Marginal Value & Binding Hours: which network limits were binding
• Data sources & coverage: what’s covered, and the update cycle