Curtailment: Generation Deliberately Reduced

An array that could produce more than it is producing, for reasons that have nothing to do with the weather or the equipment, is being curtailed. It is increasingly common as more generation connects to networks that were built for one-way flow, and it is worth understanding before a system is designed rather than after.

Three Different Causes

Export limits. The connection permits only a certain amount of power to be sent to the grid. Beyond that the system must reduce output or divert it. This is a condition of connection imposed where the local network cannot accept unlimited export.

Economic curtailment. Where generation is exposed to wholesale prices, periods of abundant supply can produce prices at which generating is not worthwhile. Large generators reduce output; small systems are usually insulated from this by the way their export is valued.

System curtailment. The network operator reduces generation because it cannot be accommodated: transmission constraints, minimum demand conditions, or stability requirements. This affects utility-scale plant far more than distributed systems.

For a residential or commercial rooftop system, the first is the one that matters.

Export Limits in Practice

A limit is enforced at the connection point rather than at the inverter. The system may generate whatever the site consumes plus the permitted export.

That distinction is the whole economics of it. A site with substantial daytime load exports little anyway, so a limit costs little. A site whose output was going to be exported is affected in proportion.

Enforcement requires metering at the connection point and control that can reduce inverter output within a short period. That equipment is part of the installation where a limit applies, and it has to be commissioned and tested rather than assumed.

What to Do With the Surplus

Where a limit would otherwise waste generation, the surplus can be used rather than curtailed.

Storage absorbs midday output for use later, which converts a limit from a loss into a timing question. Diverting to a controllable load, such as water heating or vehicle charging, does the same without a battery where such a load exists. Shifting flexible consumption into generating hours reduces the surplus in the first place.

All three convert output that would have been curtailed into output that was used, and each suits a different site.

Clipping Is Not Curtailment

These get conflated and they are unrelated.

Clipping occurs when module capacity exceeds inverter capacity, so output above the inverter’s rating is not converted. It is a deliberate design choice: oversizing the array relative to the inverter raises output across the whole year, at the cost of losing the tops of the brightest days.

It is predictable, it is chosen by the designer, and for most systems the annual trade is favourable. See sizing an inverter.

Curtailment is imposed from outside and varies with conditions the owner does not control. Different mechanism, different cause, different response.

Finding Out What Applies

Export rules are set by the utility and the regulator, so they vary by utility and by state and general statements are unreliable.

The interconnection process is where this is established. Ask during the application what export limits apply, whether they are fixed or dynamic, and what control equipment is required. The connection agreement records the answer.

Some networks operate dynamic limits, varying the permitted export with local conditions rather than fixing it. That requires communication between the system and the network operator and is more favourable than a fixed cap, since it allows full export most of the time.

See interconnection for the process.

What People Get Wrong About This

The first mistake is designing a system and discovering the export limit at the interconnection stage. It should be established first, because it changes the sizing.

The second is confusing clipping with curtailment, and treating a deliberate design choice as an imposed loss.

A third is assuming a limit makes a system uneconomic. It affects exported output only, and a site that consumes what it generates is barely affected.

A fourth is not testing the export control at commissioning. It is a control function that has to work, and a system exceeding its permitted export can be disconnected.

And the last is applying another region’s rules. These are local, they change, and the connection agreement is the only document that applies to your installation.

Related

Net metering covers how exported energy is valued where export is permitted, and grid-tied systems covers the connection this all sits within.