IEC TS 61724-3:2016

Photovoltaic system performance - Part 3: Energy evaluation method

IEC TS 61724-3:2016(E) defines a procedure for measuring and analyzing the energy production of a specific photovoltaic system relative to expected electrical energy production for the same system from actual weather conditions as defined by the stakeholders of the test. The energy production is characterized specifically for times when the system is operating (available); times when the system is not operating (unavailable) are quantified as part of an availability metric. The aim of this technical specification is to define a procedure for comparing the measured electrical energy with the expected electrical energy of the PV system.
The contents of the corrigendum of February 2018 have been included in this copy.

General Information

• Status: Published
• Publication Date: 18-Jul-2016
• ICS: 01.040.33 - Telecommunications. Audio and video engineering (Vocabularies)
• ICS: 27.160 - Solar energy engineering
• Technical Committee: TC 82 - Solar photovoltaic energy systems
• Drafting Committee: WG 3 - TC 82/WG 3
• Current Stage: PPUB - Publication issued
• Start Date: 19-Jul-2016
• Completion Date: 31-Oct-2016

Relations

• Replaces: IEC 61724:1998 - Photovoltaic system performance monitoring - Guidelines for measurement, data exchange and analysis
• Effective Date: 05-Sep-2023
• Corrected By: IEC TS 61724-3:2016/COR1:2018 - Corrigendum 1 - Photovoltaic system performance - Part 3: Energy evaluation method
• Effective Date: 05-Sep-2023

Overview

IEC TS 61724-3:2016 - Photovoltaic system performance, Part 3: Energy evaluation method defines a standardized procedure to measure and analyze the electrical energy production of a specific photovoltaic (PV) system and compare it with the expected electrical energy based on actual weather conditions agreed by stakeholders. The Technical Specification covers how to characterize energy during times the system is available and how to quantify periods of unavailability as an availability metric. (The February 2018 corrigendum is included in this edition.)

Key Topics and Requirements

• Scope and test planning
  • Define test boundary, schedule and duration (commonly a full year) to capture seasonal/weather variability.
• Predicted, expected and measured energy
  • Document initial predicted energy and methods; use measured meteorological inputs to calculate the expected energy for the test period.
• Model inputs and calculations
  • Specify meteorological inputs (irradiance, ambient temperature, wind as applicable) and PV system inputs (module, inverter, mounting).
  • Define the modelling approach and uncertainty bounds before testing.
• Data acquisition and quality control
  • Requirements for equipment, measurement intervals, time-stamp alignment and sensor calibration.
  • Procedures to identify erroneous or missing data, shading of sensors, data substitution, and handling of curtailment or grid events.
• Performance metrics
  • Calculation of energy performance index, availability, capacity factor, performance ratio, and uncertainty analysis.
• Documentation and reporting
  • Test procedure documentation and a formal test report describing methods, inputs, results, and uncertainty.

Applications and Who Uses It

IEC TS 61724-3 is used by:

• PV system owners and operators for verifying long‑term energy production.
• O&M teams and performance engineers to diagnose underperformance and optimize operations.
• Investors, asset managers and lenders for due diligence and performance guarantees.
• Certification bodies and regulators for compliance benchmarking.
• Researchers and consultants performing site-specific energy evaluations and comparisons against modeled expectations.

Practical uses include validating performance guarantees, informing maintenance prioritization, supporting project financing, and benchmarking fleet performance across weather-normalized conditions.

Related Standards

• IEC 61724-1 (PV performance data definitions)
• IEC TS 61724-2 (power measurements)
• IEC 62446 (system commissioning and verification)
• IEC 62670-2 (CPV energy measurement)
• ASTM E2848-11 (short-term power capacity measurement)

Keywords: IEC TS 61724-3, photovoltaic system performance, energy evaluation method, PV energy evaluation, expected vs measured energy, performance ratio, availability, energy performance index.

Standards Content (Sample)

IEC TS 61724-3 ®
Edition 1.0 2016-07
TECHNICAL
SPECIFICATION
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Photovoltaic system performance –
Part 3: Energy evaluation method
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IEC TS 61724-3 ®
Edition 1.0 2016-07
TECHNICAL
SPECIFICATION
colour
inside
Photovoltaic system performance –

Part 3: Energy evaluation method

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.160 ISBN 978-2-8322-3531-7

– 2 – IEC TS 61724-3:2016  IEC 2016
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references. 8
3 Terms and definitions . 8
4 Test scope, schedule and duration . 11
5 Equipment and measurements . 11
6 Procedure . 12
6.1 Overview. 12
6.2 Calculation and documentation of predicted energy and the method that will
be used to calculate the expected energy . 14
6.2.1 General . 14
6.2.2 Definition of test boundary to align with intended system boundary . 14
6.2.3 Definition of the meteorological inputs used for the prediction . 15
6.2.4 Definition of the PV inputs used for the prediction . 15
6.2.5 Definition of measured data that will be collected during the test . 16
6.2.6 Definition of the model calculations . 17
6.2.7 Predicted energy for the specified system and time period . 18
6.2.8 Uncertainty definition . 18
6.3 Measurement of data. 19
6.4 Identification of data associated with unavailability . 19
6.5 Identification of erroneous data and replacement or adjustment of such data
and preparation of model input dataset . 19
6.5.1 General . 19
6.5.2 Data checks for each data stream . 20
6.5.3 Shading of irradiance sensor . 20
6.5.4 Calibration accuracy . 21
6.5.5 Final check . 21
6.5.6 Using data from multiple sensors . 21
6.5.7 Substitution of back-up data for erroneous or missing data . 22
6.5.8 Out-of-range data or data that are known to be incorrect . 22
6.5.9 Missing data . 22
6.5.10 Partially missing data or partial unavailability . 22
6.5.11 Curtailment because of external requirement . 23
6.5.12 Inverter clipping (constrained operation) . 23
6.5.13 Planned outage or force majeure . 23
6.5.14 Grid support events (e.g. deviation from unity power factor) . 23
6.6 Calculation of expected energy . 23
6.6.1 General . 23
6.6.2 Measure inputs . 24
6.6.3 Acceptability of data . 24
6.6.4 Time interval consistency . 24
6.6.5 Time stamp alignment . 24
6.6.6 Calculate expected energy during times of unavailability . 24
6.6.7 Calculate expected energy during times of availability . 24
6.6.8 Calculate total expected energy . 24
6.6.9 Analyse discrepancies . 24

6.7 Calculation of measured energy . 25
6.8 Calculation of metrics from measured data . 25
6.8.1 Calculation of energy performance index and availability . 25
6.8.2 Calculation of capacity factor. 25
6.8.3 Calculation of performance ratio . 26
6.9 Uncertainty analysis . 26
7 Test procedure documentation . 27
8 Test report. 27
Annex A (informative) Example calculation – Calculations for the energy performance
indices . 29
Bibliography . 30

Figure 1 – Schematic showing relationship of predicted, expected, and measured
energies to reflect how the model is applied consistently to historical and measured
weather data . 14

Table 1 – Example PV performance input parameters to the model for the initial
prediction . 15
Table 2 – Example table documenting the meteorological and other input parameters
to the model for the calculation of the expected energy . 17
Table 3 − Example of data filtering criteria, to be adjusted according to local conditions . 20
Table A.1 – Fictitious data to demonstrate calculation . 29

– 4 – IEC TS 61724-3:2016  IEC 2016
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PHOTOVOLTAIC SYSTEM PERFORMANCE –

Part 3: Energy evaluation method

FOREWORD
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The main task of IEC technical committees is to prepare International Standards. In
exceptional circumstances, a technical committee may propose the publication of a technical
specification when
• the required support cannot be obtained for the publication of an International Standard,
despite repeated efforts, or
• the subject is still under technical development or where, for any other reason, there is the
future but no immediate possibility of an agreement on an International Standard.
Technical specifications are subject to review within three years of publication to decide
whether they can be transformed into International Standards.
IEC TS 61724-3, which is a technical specification, has been prepared by IEC technical
committee 82: Solar photovoltaic energy systems.
IEC 61724-1, IEC TS 61724-2 and IEC TS 61724-3 cancel and replace the first edition of
IEC 61724, issued in 1998, and constitute a technical revision.

The main technical changes with regard to the first edition of IEC 61724 (1998) are as follows:
– This first edition of IEC TS 61724-3 provides a method for quantifying the annual energy
generation for a PV plant relative to that expected for the measured weather.
The text of this technical specification is based on the following documents:
Enquiry draft Report on voting
82/1069/DTS 82/1121/RVC
Full information on the voting for the approval of this technical specification can be found in
the report on voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 61724 series, published under the general title Photovoltaic
system performance, can be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC website under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• transformed into an International standard,
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.
The contents of the corrigendum of February 2018 have been included in this copy.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
– 6 – IEC TS 61724-3:2016  IEC 2016
INTRODUCTION
The performance of a PV system is dependent on the weather, seasonal effects, and other
intermittent issues, so demonstrating that a PV system is performing as predicted requires
determining that the system functions correctly under the full range of conditions relevant to
the deployment site. IEC 62446 describes a procedure for ensuring that the plant is
constructed correctly and powered on properly by verification through incremental tests, but
does not attempt to verify that the output of the plant meets the design specification.
IEC 61724-1 defines the performance data that may be collected, but does not define how to
analyze that data in comparison to predicted performance. IEC TS 61724-2 and
ASTM E2848-11 describe methods for determining the power output of a photovoltaic system,
and are intended to document completion and system turn on, and report a short term power
capacity measurement of a PV system, but are not intended for quantifying performance over
all ranges of weather or times of year. IEC 62670-2 also describes how to measure the
energy from a CPV plant, but does not describe how to compare the measured energy with a
model.
The method described in this Technical Specification is intended to address testing of a
specific deployed PV system over the full range of relevant operating conditions and for a
sustained time (generally a complete year) to verify long-term expectations of energy
production to capture all types of performance issues, including not only response to different
weather conditions, but also outages or instances of reduced performance of the plant that
may arise from grid requirements, operational set points, hardware failure, poor maintenance
procedures, plant degradation, or other problems. The performance of the system is
characterized both by quantifying the energy lost when the plant is not functioning
(unavailable) and the extent to which the performance meets expectations when it is
functioning.
Multiple aspects of PV system performance are dependent on both the weather and the
system quality, so it is essential to have a clear understanding of the system being tested. For
example, the module temperature is primarily a function of irradiance, ambient temperature,
and wind speed; all of which are weather effects. However, the module-mounting
configuration also affects the module temperature, and the mounting is an aspect of the
system that is being tested. This technical specification presents a best-practice process for
test development and clarifies how measurement choices can affect the outcome of the test
so that users can benefit from streamlined test design with consistent definitions, while still
allowing flexibility in the application of the test so as to accommodate as many unique
installations as possible.
IECRE’s Annual PV Project Performance Certificate incorporates measurements from this
Technical Specification. Although this technical specification allows application in multiple
ways, to maintain a consistent definition of the meaning of the IECRE certificate, when this
technical specification is used for measurements for IECRE reporting, the method may be
required to use a minimum level of accuracy for the measurements or other details as
documented by IECRE.
PHOTOVOLTAIC SYSTEM PERFORMANCE –

Part 3: Energy evaluation method

1 Scope
This part of IEC 61724, which is a Technical Specification, defines a procedure for measuring
and analyzing the energy production of a specific photovoltaic system relative to expected
electrical energy production for the same system from actual weather conditions as defined by
the stakeholders of the test. The method for predicting the electrical energy production is
outside of the scope of this technical specification. The energy production is characterized
specifically for times when the system is operating (available); times when the system is not
operating (unavailable) are quantified as part of an availability metric.
For best results, this procedure should be used for long-term performance (electrical energy
production) testing of photovoltaic systems to evaluate sustained performance of the system
over the entire range of operating conditions encountered through the duration of the test
(preferably one year). Such an evaluation provides evidence that long-term expectations of
system energy production are accurate and covers all environmental effects at the site. In
addition, for the year, unavailability of the system (because of either internal or external
causes) is quantified, enabling a full assessment of electricity production.
In this procedure, inverter operation and other status indicators of the system are first
analyzed to find out whether the system is operating. Times when inverters (or other
components) are not operating are characterized as times of unavailability and the associated
energy loss is quantified according to the expected energy production during those times. For
times when the system is operating, actual photovoltaic system energy produced is measured
and compared to the expected energy production for the observed environmental conditions,
quantifying the energy performance index, as defined in IEC 61724-1. As a basis for this
evaluation, expectations of energy production are developed using a model of the PV system
under test that will serve as the guarantee or basis for the evaluation and is agreed upon by
all stakeholders of the project. Typically, the model is complex and includes effects of shading
and variable efficiency of the array, but the model can also be as simple as a performance
ratio, which may be more commonly used for small systems, such as residential systems.
The procedure evaluates the quality of the PV system performance, reflecting both the quality
of the initial installation and the quality of the ongoing maintenance and operation of the plant,
with the assumption and expectation that the model used to predict performance accurately
describes the system performance. If the initial model is found to be inaccurate, the design of
the system is changed, or it is desired to test the accuracy of an unknown model, the model
may be revised relative to one that was applied earlier, but the model should be fixed
throughout the completion of this procedure.
The aim of this technical specification is to define a procedure for comparing the measured
electrical energy with the expected electrical energy of the PV system. The framework
procedure focuses on items such as test duration, data filtering methods, data acquisition,
and sensor choice. To reiterate, the procedure does not proscribe a method for generating
predictions of expected electrical energy. The prediction method and assumptions used are
left to the user of the test. The end result is documentation of how the PV system performed
relative to the energy performance predicted by the chosen model for the measured weather;
this ratio is defined as the performance index in IEC 61724-1.
This test procedure is intended for application to grid-connected photovoltaic systems that
include at least one inverter and the associated hardware.

– 8 – IEC TS 61724-3:2016  IEC 2016
This procedure is not specifically written for application to concentrator (> 3X) photovoltaic
(CPV) systems, but may be applied to CPV systems by using direct-normal irradiance instead
of global irradiance.
This test procedure was created with a primary goal of facilitating the documentation of a
performance guarantee, but may also be used to verify accuracy of a model, track
performance (e.g., degradation) of a system over the course of multiple years, or to document
system quality for any other purpose. The terminology has not been generalized to apply to all
of these situations, but the user is encouraged to apply this methodology whenever the goal is
to verify system performance relative to modeled performance. Specific guidance is given for
providing the metrics requested for the IECRE certification process, providing a consistent
way for system performance to be documented.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
IEC 61724-1, Photovoltaic system performance – Part 1: Monitoring
IEC TS 61836, Solar photovoltaic energy systems – Terms definitions and symbols
ISO/IEC Guide 98-1:2009, Uncertainty of measurement – Part 1: Introduction to the
expression of uncertainty in measurement
ISO/IEC Guide 98-3:2008, Uncertainty of measurement – Part 3: Guide to the expression of
uncertainty in measurement
ISO 5725 (all parts), Accuracy (trueness and precision) of measurement methods and results
ISO 8601:2004, Data elements and interchange formats – Information interchange –
Representation of dates and times
ASME, Performance test codes 19.1
ASTM G113 – 09, Standard terminology relating to natural and artificial weathering tests of
nonmetallic materials
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 61724-1, ASTM
G113, IEC TS 61836, and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
______________
To be published.
3.1
energy availability
metric of energy throughput capability that quantifies the expected energy when the system is
operating relative to the total expected energy
Note 1 to entry: The energy availability is calculated from the energy unavailability and may be expressed as a
percentage or a fraction.
3.2
energy unavailability
metric that quantifies the energy lost when the system is not operating (as judged by an
automatic indication of functionality such as the inverter status flag indicating that the inverter
is actively converting DC to AC electricity or not). The energy unavailability is the ratio of the
expected energy (as calculated from the original model and the measured weather data) that
cannot be delivered because of inverters or other components being off line divided by the
total expected energy for the year
Note 1 to entry: The energy unavailability may be expressed as a percentage or a fraction. Energy unavailability
may be caused by issues either internal or external to the PV system as defined by those applying the test.
3.3
external-cause-excluded energy availability
metric that quantifies the expected energy when the system is operating relative to the total
expected energy during times when it was possible for the plant to be operating
Note 1 to entry: Exclusions are made for times when the grid is not operating or for other times when the plant
was not operating for reasons outside of the control of the plant.
3.4
predicted energy
energy generation of a PV system that is calculated with a specific performance model, using
historical weather data that is considered to be representative for the site, whereby the
specific performance model has been agreed to by all stakeholders to the test (see Figure 1)
Note 1 to entry: The historical weather data may be gathered from a weather station that is within reasonable
proximity to the site.
3.5
expected energy
energy generation of a PV system that is calculated with the same specific performance
model as that used in the predicted energy model, using actual weather data collected at the
site during operation of the system for the year in question
Note 1 to entry: The weather data is collected locally at the site.
Note 2 to entry: The expected energy is used to calculate the energy performance index.
3.6
measured energy
electric energy that is measured to have been generated by the PV system during the test
over the same duration as the expected energy model
Note 1 to entry: See also 3.13 test boundary to define the location of measurement.
3.7
performance index
electricity generation of a PV system relative to expected, as defined in IEC 61724-1 and
calculated as described in this technical specification
3.8
energy performance index
electricity generation of a PV system relative to the expected energy over a specified time
period, as defined in IEC 61724-1 and calculated in this technical specification. The energy

– 10 – IEC TS 61724-3:2016  IEC 2016
performance index may refer to all times or only times of availability as defined by the all-in
energy performance index or the in-service energy performance index, respectively
3.9
all-in energy performance index
electricity generation of a PV system relative to the total expected energy over a specified
time period, including times when the system is not functioning
3.10
in-service energy performance index
electricity generation of a PV system relative to the expected energy over a specified time
period during times when the system is functioning (excluding times when inverters or other
components are detected to be off line)
3.11
power performance index
electricity generation of a PV system relative to expected power production for a specified set
of conditions, as defined in IEC 61724-1 and calculated as in IEC TS 61724-2
3.12
primary sensor
sensor that has been designated as the source of data for the test. Primary sensors may be
designated for the irradiance, temperature, wind speed or other measurements. The electrical
measurements are defined as part of the system definition
3.13
test boundary
a (physical) differentiation between what is considered to be part of the system under test and
what is outside of the system for purposes of quantifying the performance index
Note 1 to entry: Quantification of the energy unavailability may be affected by events outside of the test
boundary.
3.14
stakeholders of the test
individuals or companies that are applying the test
Note 1 to entry: Commonly, these parties may be the PV customer and the PV installer, with the test method
applied to define completion of a contract, but the test method may be applied in a variety of situations and the
stakeholders of the test may in some cases be a single individual or company.
3.15
test
test that compares the measured output of a PV system over a prolonged time period to the
output that was expected for the PV system for the measured set of weather conditions, as
defined by this technical specification (see 3.4)
3.16
model
simulation model used to calculate both predicted and expected PV generation from weather
data. The model is also used to calculate expected energy during times of unavailability
Note 1 to entry: Typically, the model is expected to be the same that was used to describe the plant before
construction, but the model may be updated to reflect changes in the plant design, or any model may be used if the
goal is to test the accuracy of the model. It is assumed that the model is appropriate for the situation.
3.17
inverter clipping
the inverter output is limited by the capability of the inverter rather than by the input power
from the PV array
4 Test scope, schedule and duration
This test may be applied at one of several levels of granularity of a PV plant. The users of the
test shall agree upon the level(s) at which the test will be applied. The smallest level to which
the test may be applied is the smallest AC power generating assembly capable of
independent on-grid operation.
PV plant construction is often divided into phases. Phases may have separate or shared
interconnection points and may be spread over a period of months or even years. In general,
it is recommended that the test be applied at the highest level, that which encompasses the
entire PV project. However, for very large plants scheduled for interconnection in parts, with
the first and last interconnection separated by a period of more than 6 months. It is
recommended that the test be applied to smaller subsets of the plant as they become
available for interconnection. In such cases, upon full plant completion the test may be
applied again in a way that encompasses the entire plant, but in these cases the expected
energy is modified to include expected plant performance degradation in accordance with the
model accepted by the stakeholders of the test.
Some PV modules show measurable performance changes within hours or days of being
installed in the field, others do not. The start of the test should be negotiated between the
stakeholders using the manufacturer’s guidance for the number of days or the irradiance
exposure needed for the plant to reach the modeled performance along with the details of the
actual installation and interconnection dates. Any degradation assumptions should be agreed
to by all stakeholders and documented as part of the model description.
It is recommended that the test lasts 365 days. The actual test term should be agreed upon in
advance. If the test is not continued for a full year, seasonal variations (including shading,
spectrum, temperature, and wind) may cause the performance to deviate from what would be
obtained over a full year.
The performance metric, in-service energy performance index, is reported only for times when
the inverters and other components are on line. Expected energy for times when the inverters
or other components are off line is quantified in the energy unavailability metric. The energy
unavailability metric may be further divided into situations with internal and external causes,
as agreed to by the stakeholders.
All stakeholders agree on a detailed test procedure before the test commences as described
in Clauses 5 and 6.
5 Equipment and measurements
Using the default test boundary (used for simplified discussion here), the weather is
characterized by:
• Global horizontal irradiance (direct and diffuse may also be measured).
• Ambient temperature.
• Wind speed.
• Rainfall or soiling (if the test agreement assumes a clean system).
If additional characterization of the weather is required for implementation of the model, these
data shall be collected in a manner consistent with the model. If the model uses a different
test boundary then the default test boundary is modified. For example, if plane-of-array
irradiance is specified as an input to the model, defining the albedo to be outside of the test
boundary, then the weather is characterized by the plane-of-array irradiance rather than the
global horizontal irradiance.
– 12 – IEC TS 61724-3:2016  IEC 2016
Some models use other inputs such as atmospheric pressure and humidity since these can
affect the incident light spectrum and the PV performance. Whereas it is encouraged to
monitor many aspects of the PV system operation to best understand the status of the system
and optimize its performance, the use of data from the system as a characterization of the
weather inputs to the model risks compromising the integrity of the test. When data are used
for such characterization there is the risk that some aspects of the system performance are
then considered to be part of the uncontrolled weather. For example, if modules are mounted
without adequate ventilation, the temperature of the system may increase beyond the design
value, reducing system output. Similarly, a tracked system that does not track correctly will
measure a plane-of-array irradiance that is lower than what it would have been with optimal
tracking. Although frequent rain and snow will affect system performance, the design of the
system may aid in shedding snow and/or being resistant to soiling.
The system output is characterized by:
• Real AC power delivered to the grid.
• Apparent AC power or AC power factor.
The model simulating the PV system performance should include an assumption about the
power factor, which may affect the predicted energy. The recorded power factor (or any
similar input to the model) should be then used when calculating the expected energy, as
described below.
The definition of the AC energy, including the point of measurement (such as at a utility-grade
meter at the point of interconnection) is documented as part of the test boundary definition. If
parasitic loads outside the system boundary exist (e.g., trackers and night-time electricity use
by inverters and transformers), the contract or test definition defines whether adjustments are
made for these, and, if so, these adjustments are characterized.
Measurement equipment and procedures for all measured parameters are recommended to
conform to IEC 61724-1, Class A requirements. However, a Class B or Class C evaluation
(per the contract) may also be completed and documented in the final report.
All details of data collection (including sensor number, maintenance, calibration and cleaning)
follow IEC 61724-1 according to the chosen Class of measurement with the exception of:
• The choice of sensor and sensor positioning shall be consistent with the performance
model that is being used for the test.
NOTE Often the final uncertainty of the measurement is dominated by the uncertainty of the irradiance
measurement, so high-accuracy sensors are desired.
• The frequency of cleaning of irradiance sensors may vary by site and should be
documented.
• Verification of accurate positioning of the sensors is accomplished through comparison of
data from a clear day with modelled irradiance for a clear day and the results included in
the documentation of the uncertainty of the application of the test.
• When irradiance sensors are deployed in the plane of the array, the ground albedo should
be measured to demonstrate consistency with that assumed in the model and the results
included in the documentation of the uncertainty of the application of the test.
• For Class A tests, because the irradiance measurement is so crucial to the test, the
calibrations should be independently verified either by using sensors calibrated at different
test locations or at different times so as to prevent a systematic bias to the calibration.
6 Procedure
6.1 Overview
The terms “predicted” and “expected” energy are defined in 3.4 and 3.5 to avoid ambiguity
when differentiating the prediction based on historical weather data from the prediction based

on the measured weather data for the time of interest. The methods used for calculating the
“predicted” and “expected” energies are aligned for consistency. If the historical and
measured weather data differ in their format, the applied model may be inadvertently
changed. Care shall be taken to address the differences in the weather data used for the two
calculations so that the model used for calculating the “predicted” energy is the same as the
model used for calculating the “expected” energy.
The comparison of measured energy to expected energy is simplified by collecting the new
weather data in the same format as the historical data. In this case both parties agree upon
and document data in an identical format.
The comparison of the modeled and test results to evaluate the energy performance index is
documented in detail in the following subclauses. The following list summarizes 6.2 to 6.9:
• Define test boundary to align with the intended system boundary.
• Calculate and document the predicted energy using the chosen model by listing all inputs
including historical weather data, assumptions regarding soiling, shading, outages, etc.;
the raw data should be included in the final report as an appendix. The predicted energy
may assume 100 % availability or may be reduced to account for expected times of
unavailability.
• Complete the measurement of data from the operating system over the test period.
• Identify times when the system is unavailable for a variety of reasons that may be external
or internal to the plant.
• Evaluate the measured data to identify and document anomalies that may require extra
treatment. Such anomalies include missing or erroneous data that are replaced.
• Calculate and aggregate the expected energy for the full time period, replacing missing
data, as needed.
• Aggregate the measured energy, replacing missing data, as needed.
• Compare the expected and measured energies from the plant to derive the energy
performance index.
• Compute the uncertainty of the measurement.

– 14 – IEC TS 61724-3:2016  IEC 2016

Setting the Predicted value
Fixed model inputs
Output
Model
(predicted)
Variable inputs
(meteorological, HISTORICAL)
Generating the Expected value
Same fixed model inputs
Output
Same model
(expected)
Variable inputs
(meteorological, MEASURED)
Generating the Measured value
Output of plant (measured)
IEC
Times of unavailability are not addressed in this figure.
Figure 1 – Schematic showing relationship of predicted, expected,
and measured energies to reflect how the model is applied consistently
to historical and measured weather data
6.2 Calculation and documentation of predicted energy and the method that will be
used to calculate the expected energy
6.2.1 General
As shown in Figure 1, the first step in the process, typically, is to predict the performance of
the PV system based on historical weather data using a model that has been agreed to by the
stakeholders. The model is defined in terms of the model inputs, calculation process, and how
the measured meteorological data will be input into the model. It is expected that the
information required per this subclause (6.2) is documented before the beginning of the test;
although the final comparison of expected and measured energy does not use the predicted
energy directly, the predicted energy is usually required for project planning. The model may
assume 100 % availability or may specify a predicted unavailability as part of the prediction,
reducing the predicted energy for the year accordingly.
6.2.2 Definition of test boundary to align with intended system boundary
This test method is intended to quantify the performance of a system, but the result of the test
may depend on what is considered to be part of the system. The stakeholders of the test shall
agree on the definition of the system including:
• The meter(s) that define the output of the system.

• Aspects of system design that are being tested such as whether modules are mounted
according to the design (tilt, azimuth, height, racking design) all
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