Performance ratio calculation

Performance ratio (PR) answers the question every operator asks: “Is my plant performing as it should, given the sun we had today?” It normalises away the weather so you can compare performance across days, plants, and seasons on a fair basis.

Before comparing two PR values, match both the PR variant and the period. A current value, an analytical daily result, and an accumulated monthly result each describe a different scope — treating them as interchangeable is the most common source of confusion.

How the measurements relate

flowchart LR
    A[Irradiance] -->|integrated over time| B[Irradiation]
    C[Power] -->|integrated over time| D[Yield]
    B --> E[Base PR]
    D --> E
    F[Plant normalization factor] --> E
    G[Module temperature inputs] --> H[Corrected PR]
    E --> H
    I[Estimated curtailed energy] --> J[Overspill PR]
    I --> K[Total PR]
    D --> K

Not every PR variant is available for every plant or view. The table below defines the raw terms the diagram connects.

Check the plant’s PR calculation profile in Plant Information to see whether the calculation uses device readings or calculated inputs. The inverter and sensor device tabs also show compact PR input indicators. These labels identify whether weather stations contribute equally or through configured weights and which fallback applies when a selected device reading is unavailable.

Term	Meaning	Typical unit
Irradiance	Solar power density at a point in time	W/m²
Irradiation	Solar energy accumulated over a period	Wh/m²
Power	The rate of electrical output	kW
Yield	Electrical energy produced over a period	kWh
Plant normalization factor	The configured factor that relates plant capability to the irradiation term	Plant-specific

💡 Irradiance vs. irradiation — they’re easy to mix up. Think of irradiance as a speedometer (instant), and irradiation as the odometer (accumulated over time). An irradiance of 600 W/m² held for five minutes contributes:

$600 \times \frac{5}{60} = 50\ \text{Wh/m}^2$

of interval irradiation. They tell you different things.

Current and analytical results

SolarSENS produces performance results in two contexts, and they can differ:

Context	What it helps answer	Typical grain	Why it can change
Current or recent	What is happening now?	Recent interval values	Updated as fresh telemetry arrives
Analytical daily or monthly	How did the completed period perform?	Daily or accumulated monthly	Updated when corrected or late inputs are reprocessed

A recent value and an analytical value for the same moment can differ because they use different windows and because corrected inputs arrive later. See Data freshness and corrections for how updates propagate.

Base, effective, and corrected PR

At its simplest, base PR compares what the plant actually produced against what it could have produced given the available sunlight:

\text{PR} = \frac{\text{Yield}} {\text{Irradiation} \times \text{Plant normalization factor}} \times 100\%

The plant normalization factor is the configuration that bridges irradiance data to your specific plant’s capability — it accounts for module area, rated capacity, and unit conversions. Get this value right during commissioning; an incorrect factor makes PR comparisons unreliable across plants.

Effective PR

Effective PR uses only intervals that meet the plant’s configured irradiance threshold. This prevents low-light hours (dawn, dusk, heavy cloud) from dragging the ratio down and making a healthy plant look weak.

⚠️ Because the qualifying threshold is configuration-dependent, only compare effective PR when you know the plants and periods use compatible settings and data coverage.

Corrected PR

Corrected PR adjusts the irradiation reference using PV module temperature and configured temperature coefficients. It is useful when comparing periods where module temperature differed significantly — summer vs. winter, or a plant with tracking vs. fixed-tilt.

This follows the industry weather-corrected PR method described in NREL/TP-5200-57991 and adopted in IEC 61724-1.

⚠️ Compare corrected PR with corrected PR, and uncorrected PR with uncorrected PR. They are not interchangeable.

Curtailment, Overspill PR, and Total PR

Sometimes the plant could have produced more, but didn’t — because of grid curtailment, inverter clipping, or active power limiting. SolarSENS captures this as curtailment (overspill): an estimate of producible energy that was withheld under qualifying operating conditions.

The estimate requires suitable plant configuration, measured production, irradiance or model inputs, and eligible conditions. It is not available for every plant or interval.

Indicator	What it represents
Overspill PR	Estimated curtailed energy, normalised by irradiation and plant capability
Total PR	Delivered energy + estimated curtailed contribution, normalised the same way

\text{Overspill PR} = \frac{\text{Estimated curtailed energy}} {\text{Irradiation} \times \text{Plant normalization factor}} \times 100\%

\text{Total PR} = \frac{\text{Delivered energy} + \text{Estimated curtailed energy}} {\text{Irradiation} \times \text{Plant normalization factor}} \times 100\%

How monthly PR is accumulated

Monthly PR is calculated from accumulated monthly energy and irradiation — it is not the arithmetic average of daily PR percentages.

This simplified example omits plant-normalization constants so it illustrates aggregation only:

Day	Energy numerator	Irradiation denominator	Daily ratio
1	80	100	80%
2	10	20	50%

The accumulated monthly ratio is:

\frac{80 + 10}{100 + 20} = 75\%

Not the daily average:

\frac{80\% + 50\%}{2} = 65\%

💡 This matters because a high-PR day with lots of sun (Day 1: 80% on 100 Wh/m²) contributes more to the total than a low-PR day with little sun (Day 2: 50% on 20 Wh/m²). Averaging the percentages would give too much weight to the low-sun day.

Industry references

These indicators follow established industry definitions:

IEC 61724-1:2021 — Photovoltaic system performance, Part 1: Monitoring
Weather-Corrected Performance Ratio (NREL/TP-5200-57991), Dierauf et al., 2013