Why curtailment is the most under-reported number in renewables

Ask an operator how much output their portfolio produced last quarter and you will get a confident answer. Ask how much it could have produced and the room goes quiet. Curtailment sits in the gap between those two numbers, and in most reporting it simply disappears.

Where the number goes

Grid instructions arrive as set-points, not as energy. A turbine told to hold at 60% for four hours does not record the 40% it was asked not to make. Unless something is measuring the resource at the same time, the lost energy never exists in any ledger, so it never reaches a settlement, a claim or a board report.

Across the portfolios we instrument, curtailed energy ranges from under 2% to just over 19% of available output. The high end is not exotic: it is a well-sited wind farm behind a constrained export line.

A constrained export line is the most common reason a well-sited farm is told to hold back.

How we measure it

Curtailment only becomes a number when two series exist side by side: what the resource made available, and what the meter says was delivered. We take both, at the same clock, and log every gap against the grid instruction that caused it.

• Resource measured at the nacelle or plane of array, not at the meter

• Available energy computed hour by hour from the measured resource

• Delivered energy read at the point of connection

• Every shortfall logged against the set-point that caused it

An hourly, instrument-level view

The fix is not a better model. It is a second measurement, taken at the resource rather than the meter, so that available and delivered energy can be compared hour by hour. With both series in hand, curtailment stops being an estimate in a footnote and becomes a line item that can be negotiated, insured or compensated.

If you operate behind a constraint, this is the most valuable number you are not reporting.