Curtailment and overspill estimation

Curtailment (overspill) is SolarSENS’ estimate of energy a plant could have produced but did not deliver because its output was limited below what operating conditions could support.

Overspill is estimated, not directly measured. The plant meter tells you what was delivered; SolarSENS uses production, irradiance, control, and—where required—grid-meter context to estimate the additional energy that may have been available under qualifying curtailment conditions.

At a high level, the calculation asks three questions:

What could the plant have produced? SolarSENS builds an expected-output baseline from qualifying operating history and measured irradiance.
Was output actually limited? Measured inverter power is compared with the expected output, together with the available active-power control signal.
Does the interval pass the required validation? Some calculation paths also use grid-meter conditions before retaining an overspill estimate.

How an interval becomes an overspill candidate

SolarSENS first establishes the expected output for the interval. The baseline can use the soiling model to estimate expected inverter output; it does not directly measure dirt or soiling on equipment.

Inverter power and operating historyIrradiance

Expected-output baseline

Expected outputMeasured power and power limitGrid-meter reading

Overspill check

Five-minute overspill estimate

The calculation uses:

Input	Role in the estimate
Inverter readings	Provide measured active power and the power-limit information used to identify a qualifying limitation.
Irradiance	Indicates how much solar input was available during the interval.
Inverter information	Associates the calculation with the relevant inverter and its rated information.
Expected-output baseline	Provides the reference output SolarSENS compares with measured production.
Grid-meter readings	Validate candidates on calculation paths that require meter context.

An interval must have a usable baseline, measured irradiance, inverter information, and an active-power control signal before it can become a candidate.

For standard inverter data, the recorded power limit must also be below full output. Low measured production by itself is not treated as curtailment.

From the power gap to overspill energy

For a qualifying candidate interval, SolarSENS calculates only the positive difference between expected and measured inverter output:

Estimated overspill power = max(expected output − measured active power, 0)

The retained power estimate is then converted into energy for the five-minute interval:

Estimated overspill energy = estimated overspill power × 5 ÷ 60

For example, an estimated 300 kW power gap sustained for one five-minute interval corresponds to 25 kWh of estimated overspill energy.

Interactive example

Example daily readings

Thirty-minute readings for an example day. Hover, focus, or use the arrow keys on the chart to inspect an interval.

Showing all series. The left axis is power in kW; the right axis is irradiance in W/m².

Use the interactive example to change the expected output, measured power, grid power, model reference, and Config Server threshold. The example applies a chart-wide adjustment based on those readings while preserving the daily pattern. It illustrates the calculation logic only; it is not a live plant result.

Meter-validation conditions

Some calculation paths apply an additional meter check after an overspill candidate has been established. On those paths, SolarSENS retains the estimate when either of the following conditions is met:

Condition	What SolarSENS checks
High consumption	Grid power is below the plant’s configured share of the combined active-power and grid-power readings: `grid power ÷ (active power + grid power)`. The default threshold is 5% and can be changed for the plant in Config Server.
Low consumption	A model-derived output reference, reduced by 5%, is greater than the combined active-power and grid-power readings: `95% of model-derived reference > active power + grid power`.

These checks are validation gates, not the overspill formula itself. If neither condition is met on a path that requires meter validation, SolarSENS does not retain the estimate for that interval.

How to interpret overspill

Overspill is most useful when read together with delivered generation and the plant’s performance indicators.

Use it to	Do not use it to
Estimate the production impact of a qualifying output limitation.	Treat the value as directly metered curtailed energy.
Understand the curtailed contribution represented by Overspill PR and Total PR.	Assume every low-production interval was curtailed.
Compare estimated curtailed energy with delivered generation for the same plant and period.	Compare values across plants or periods without matching their calculation context and configuration.

Relationship to performance ratio

SolarSENS uses estimated curtailed energy in two related performance indicators:

Overspill PR represents the estimated curtailed contribution, normalized against the same irradiation and plant capability context used for PR.
Total PR considers delivered energy together with the estimated curtailed contribution.

See Performance ratio calculation for the PR formulas and guidance on comparing PR variants.