Curtailment Explained

Curtailment means deliberately producing less electricity from a plant than it could make at that moment. The term is used most often for wind and solar, where the fuel is free and any output not taken is simply lost. It happens when the grid physically cannot move the power to where it is needed, or when there is more electricity on offer than anyone wants to consume.

Why a free resource gets turned down

Electricity has an awkward property: supply and demand must match continuously, second by second, across the whole network. There is no warehouse. If generators put more onto the grid than consumers take off, frequency rises and equipment is at risk, so somebody has to produce less.

Normally the market sorts this out through price. The most expensive plants stop running first. But wind and solar have almost no running cost, so when the cheap sources alone can cover demand, the adjustment has to come from them. That is the moment curtailment begins.

There are two broad causes, and they call for different fixes.

Congestion curtailment

The first cause is that the wires are full. Transmission lines have thermal limits, and a line carrying its maximum cannot carry more no matter how much generation sits behind it.

Wind and solar farms tend to be built where the resource is good and land is cheap, which is often far from cities. The grid connecting those places was frequently built for a much smaller flow, sometimes for nothing more than serving a few farms. When a large wind fleet is producing hard, the export path out of that region fills up, and the operator instructs some plants to reduce output to keep flows within limits.

This kind of curtailment is very local. One region can be curtailing heavily while the country as a whole is short of power. It is a wires problem, not a supply problem, and the answers are wires answers: reinforce the line, reconfigure the network, install equipment that lets existing lines carry more, or connect new plants where capacity already exists.

Oversupply curtailment

The second cause is that the system as a whole has more generation than it needs.

Sunny spring weekends are the classic case. Demand is low because heating and cooling are both off and many businesses are closed, while solar output is near its best. Add strong wind overnight and the surplus can last for hours. Systems with a large solar fleet meet this regularly: Ember’s statistics show that Chile generated about a quarter of its electricity from solar in 2025, and the middle of the day in such a system looks very different from the evening.

Oversupply is made worse by generation that cannot easily be turned down. Some thermal plants have minimum stable levels below which they must shut down entirely, and restarting takes hours and costs money. Others must keep running to provide services the grid needs, or because they supply heat to a district heating network or an industrial process. Nuclear plants in some countries can follow load, in others they run flat. The output that cannot move is often called must run generation, and the more of it there is, the less room is left for wind and solar.

When wholesale prices are set by a market, this situation usually shows up as very low or negative prices before it shows up as an instruction from the operator. Plants that lose money by running choose to stop, which is curtailment arriving by economics rather than by command.

Other reasons output gets reduced

A few less obvious causes are worth knowing about.

• System security. Operators need enough conventional plant online for voltage control, short circuit strength and inertia. Where those services still come mainly from spinning machines, a minimum number of them must run even when cheaper output is available.
• Reserve requirements. Some capacity is deliberately held back so it can increase output if something fails. A wind farm asked to run below its potential can provide that headroom, which is useful but counts as curtailment.
• Connection agreements. Some plants connect on terms that explicitly allow the operator to limit them, in exchange for connecting sooner or more cheaply than waiting for a reinforcement.
• Ramping limits. A plant may be told to reduce output not because of a surplus now, but because the system cannot handle how fast its output would otherwise change.

What reduces curtailment

No system gets to zero, and aiming for zero would be wasteful, because the last few hours of surplus each year are very expensive to accommodate. The realistic goal is keeping it modest. The main tools:

Transmission. New lines and upgrades to existing ones are the direct fix for congestion, though they take years to plan and build.

Storage. Batteries absorb midday surpluses and release them into the evening peak, which turns curtailed energy into useful energy and improves the economics of both. Storage is best at moving power by hours, not by weeks.

Interconnection. Links to neighbouring grids export a surplus instead of wasting it, as long as the neighbour is not in surplus at the same time.

Flexible demand. Electric vehicle charging, water heating, hydrogen electrolysis, desalination and industrial processes that can choose their hours all convert surplus power into something valuable. Time varying tariffs are what make that happen at scale.

Market and network rules. How curtailment is allocated, whether plants are paid for it, and whether they can connect on flexible terms all shape how much occurs and who bears the cost.

Where plants are built. Siting new wind and solar with network capacity in mind, and mixing technologies that peak at different times, avoids creating congestion in the first place.

How to read curtailment figures

Curtailment is usually reported as the share of potential output that was not produced, for example a percentage of what a region’s wind fleet could have generated over a year. Take care with comparisons. Some figures count only instructions from the operator, others also count output withdrawn in response to prices. Some cover a single congested region, others a whole country. A region with a high percentage may be curtailing a small fleet, while a low percentage across a very large fleet can be more energy in absolute terms.

Rising curtailment is not automatically bad news. It often means a lot of new capacity has been built quickly, which is what a fast build out looks like before the wires, storage and flexible demand catch up. It is worth watching because it measures how well the rest of the system is keeping pace. Our grid zone pages show the hourly generation mix for US balancing authorities, which is where these patterns become visible.