In this document
This document explains the performance indicators used to evaluate the operational performance of PV power plants: Performance Ratio (PR), temperature-corrected PR, and Energy Performance Index (EPI).
Overview
Performance indicators provide a standardized, quantitative measure of how well a PV power plant is performing relative to its expected output. They are defined in IEC 61724-1:2021 and are used by asset owners, operators, and independent engineers to detect underperformance, benchmark plants, and fulfill contractual obligations.
Two primary indicators are in common use: Performance ratio (PR) and Energy performance index (EPI). PR is the established industry standard and the most widely used metric. EPI is a newer, more advanced indicator that is increasingly replacing PR in detailed performance assessments, particularly where high temporal resolution and climate independence are required.
Performance Ratio (PR)
PR measures how effectively a PV plant converts the solar radiation collected by its modules into AC energy delivered to the off-taker, relative to what would be expected from the nameplate rated capacity under STC (standard test conditions). It captures the combined effect of all losses in the system, including thermal losses, shading, soiling, component failures, and downtime.
PR is assessed over various timeframes: annual, monthly, and daily values are all common. An annual PR of 80% means the plant delivered 80% of the energy it would have produced if it always operated at its nameplate efficiency.
PR is calculated as:
where:
is the measured AC energy output over the assessment period [kWh] is the measured or modelled plane-of-array (POA) irradiation over the same period [kWh/m²] is the irradiance at STC, equal to 1000 W/m² is the nameplate DC capacity of the PV array [kWp]
Limitations
PR is sensitive to ambient temperature. In hot climates or summer months, modules operate at higher temperatures and therefore at lower efficiency, which reduces PR. This means PR values are not directly comparable across different climates or seasons without correction. PR is also susceptible to errors in solar radiation measurements. Poorly maintained instruments producing deflated irradiance readings will result in falsely inflated PR estimates.
Temperature-corrected PR
Temperature-corrected PR removes the seasonal influence of module operating temperature, making values comparable across time periods and locations.
Temperature-corrected PR is calculated as:
where:
is the measured energy output for time step j [kWh] is the nominal DC power [kWp] is the plane-of-array irradiance at time step j [W/m²] is the reference irradiance (1000 W/m²) is the temperature coefficient of the PV module [%/°C] is the measured or modelled module temperature at time step j [°C] is the reference module temperature (typically 25°C)
Note: Temperature-corrected PR requires reliable module temperature measurements or modelled temperature data. This is one reason why PV module temperature (TMOD) is recommended as a supplemental measurement parameter in well-designed meteorological stations.
Energy Performance Index (EPI)
EPI is a standardized indicator defined in IEC 61724-1:2021. It is defined as the ratio of measured specific energy yield to the expected (modelled) specific energy yield.
EPI is calculated as:
where:
is the measured specific energy yield for period j [kWh/kWp] is the expected specific energy yield for the same period, calculated using a PV simulation model
For a newly operational plant with no defects or losses beyond those modelled at pre-construction stage, EPI is expected to equal 1.0 (100%). Values below 1.0 indicate underperformance relative to model expectations.
Advantages over PR
EPI has several advantages compared to standard PR:
Climate independence: Because EPI normalizes against a modelled expected yield - which already accounts for local climate conditions - it is inherently independent of weather variability. PR is not.
Short time frame analysis: EPI can be meaningfully calculated at high temporal resolution (15-minute intervals), enabling detection of operational issues that a monthly or annual PR would mask.
Loss identification: Because expected production is calculated using detailed PV simulation, EPI analysis can identify and quantify specific categories of losses - including soiling, shading, component failures, and availability losses - by comparing modelled versus actual loss profiles.
Calibration of pre-construction estimates: EPI analysis supports revision of the long-term energy yield estimate after commissioning, by reconciling modelled assumptions with observed plant behavior.
Requirements
The main requirement for EPI calculation is a detailed, accurate PV simulation model and complete knowledge of the power plant's technical configuration. This makes EPI more demanding to implement than PR, but also more informative.
Note: Accurate solar radiation input data - ideally from quality-assessed ground measurements or site-adapted satellite data - is essential for reliable EPI calculation. Errors in irradiance data directly propagate into the expected yield and will distort the EPI result.