Performance ratio calculation

Performance ratio (PR) is a normalized way to compare plant output against the solar resource available to the plant. In SolarSENS, PR is used to separate raw energy production from the effect of irradiance, estimated overspill, and temperature correction.

Overview

This concept explains four related calculations used in SolarSENS:

Total PR for monthly plant performance including estimated overspill energy
Effective PR for performance during valid operating irradiance conditions
Overspill energy as an estimate of energy not converted into active power
Weather-corrected PR for performance normalized by module temperature effects

Why it matters

It helps teams compare plant performance across periods with different solar resource levels.
It separates energy loss interpretation into standard production, low-irradiance filtering, and overspill estimation.
It gives operators and analysts a clearer way to explain whether low PR comes from weather, temperature, curtailment-like behavior, or plant underperformance.

Core terms

Use these terms consistently when reading or documenting PR in SolarSENS:

Term	Meaning
Irradiance	Instantaneous solar power density, typically expressed in W/m².
Irradiation	Solar energy accumulated over a period, typically expressed in kWh/m².
Yield	The measured energy produced during the calculation period.
PR Factor	The plant normalization factor configured in SolarSENS and used in the PR denominator.
Theoretical power	The modeled or expected plant power for the interval.
Active power	The measured real output power for the interval.
PV gamma	The module power temperature coefficient used for weather correction.
STC temperature	The standard test condition module temperature, normally 25°C.

In the PR denominator, SolarSENS also applies a fixed scaling term of $1000 \times 100$ . Document it as a unit-normalization step:

1000 aligns the irradiation-based reference term with the plant factor’s kW-based nameplate scale used by the SolarSENS denominator.
100 converts the resulting decimal ratio into the percentage-style PR representation shown in the platform.

When documenting or validating PR, keep this scaling term consistent with the configured plant PR factor so like-for-like comparisons remain valid.

How it works in SolarSENS

Total PR

SolarSENS defines monthly total energy by adding measured monthly yield and the monthly estimated overspill energy contribution.

\text{Monthly Total Energy} = \text{Monthly Yield} + \text{Monthly Estimated Overspill Energy}

\text{Monthly Total PR} = \frac{\text{Monthly Total Energy}}{\text{PR Factor} \times 1000 \times 100 \times \text{Monthly Total Irradiation}}

Use total PR when you want the monthly indicator to reflect both delivered energy and the platform’s estimated overspill contribution.

Effective PR

Effective PR filters out very low irradiance periods. In SolarSENS, only intervals with irradiance of 50 W/m² or greater contribute to effective energy.

\text{Eff. Energy} = \sum \begin{cases} \text{Yield}, & \text{if Irradiance} \geq 50\ \text{W/m}^2 \\ 0, & \text{otherwise} \end{cases}

\text{Daily Eff. PR} = \frac{\text{Daily Eff. Energy}}{\text{PR Factor} \times 1000 \times 100 \times \text{Daily Total Irradiation}}

\text{Monthly Eff. PR} = \frac{\sum \text{Daily Eff. Energy}}{\text{PR Factor} \times 1000 \times 100 \times \text{Monthly Total Irradiation}}

This filtering is useful because PR can become unstable during dawn, dusk, and other low-light conditions. Effective PR focuses the calculation on intervals where the plant is operating under more meaningful irradiance.

Overspill energy

SolarSENS estimates overspill energy from the gap between theoretical power and active power over a 5-minute interval.

\text{Est Overspill Energy} = (\text{Theoretical Power} - \text{Active Power}) \times \frac{5}{60}

\text{Daily Overspill PR} = \frac{\text{Est Overspill Energy}}{\text{PR Factor} \times 1000 \times 100 \times \text{Daily Total Irradiation}}

This estimate is useful when SolarSENS needs to represent performance that is not visible in measured active power alone. In monthly total PR, the overspill estimate is folded back into monthly total energy.

flowchart TD
    A[Monthly Yield] --> B[Monthly Total Energy]
    C[Monthly Estimated Overspill Energy] --> B
    B --> D[Monthly Total PR]
    E[Monthly Total Irradiation] --> D
    F[PR Factor × 1000 × 100] --> D

Weather-corrected PR

The corrected PR variant follows the idea of weather-corrected PR: it uses module temperature to correct the irradiation term before calculating PR.

For this corrected version, SolarSENS includes intervals where irradiance is non-negative, then applies a temperature correction coefficient to the irradiation increment.

\text{Daily Eff. Energy} = \sum \begin{cases} \text{Yield}, & \text{if Irradiance} \geq 0\ \text{W/m}^2 \\ 0, & \text{otherwise} \end{cases}

\text{ck} = 1 + \text{PV gamma} \times (\text{PV Module Temperature} - \text{STC Temperature})

\text{Radiation Delta Corrected} = \text{ck} \times \text{Radiation Delta}

\text{Cumulative Irradiation Corrected} = \sum \text{Radiation Delta Corrected}

\text{Daily Eff. PR Corr} = \frac{\text{Daily Eff. Energy}}{\text{PR Factor} \times 1000 \times 100 \times \text{Cumulative Irradiation Corrected}}

In practice:

ck increases or decreases the irradiation contribution according to module temperature.
PV gamma is usually negative, so higher module temperature reduces the corrected reference term.
corrected PR is best interpreted as a weather-normalized performance indicator rather than a direct replacement for standard PR.

How to interpret the variants

Total PR: best when you want a monthly indicator that includes estimated overspill contribution.
Effective PR: best when you want to remove low-irradiance noise from the comparison.
Weather-corrected PR: best when you want to account for module temperature effects and compare periods more fairly.

These indicators answer different questions, so they should be compared with like-for-like definitions rather than treated as interchangeable values.