ISO/TR 9901:2021

TECHNICAL ISO/TR
REPORT 9901
Second edition
Solar energy — Pyranometers —
Recommended practice for use
Énergie solaire — Pyranomètres — Pratique recommandée pour
l'emploi
PROOF/ÉPREUVE
Reference number
ISO/TR 9901:2021(E)
©
ISO 2021

---------------------- Page: 1 ----------------------
ISO/TR 9901:2021(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2021
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii PROOF/ÉPREUVE © ISO 2021 – All rights reserved

---------------------- Page: 2 ----------------------
ISO/TR 9901:2021(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Selection of pyranometers and accessories . 4
4.1 General . 4
4.2 Pyranometer selection based on accuracy class . 4
4.3 Pyranometer and accessory selection based on other considerations . 6
4.4 Measuring system redundancy and spatial resolution . 6
4.5 Common pyranometer accessories . 7
4.5.1 Electronics, data acquisition and power supply . 7
4.5.2 Heating and ventilation systems . 8
4.5.3 Mounting stands and supports . 9
4.6 Personal safety . 9
5 Recommended practice for use .10
5.1 General .10
5.2 Pyranometers measuring plane of array and global horizontal irradiance .10
5.2.1 General.10
5.2.2 Installation .10
5.2.3 Heating and ventilation .13
5.2.4 Inspection and maintenance .13
5.2.5 Data acquisition and storage .16
5.2.6 Data quality control and correction .19
5.3 Pyranometers measuring diffuse radiation .20
5.3.1 General.20
5.3.2 Installation .22
5.3.3 Heating and ventilation .23
5.3.4 Inspection and maintenance .23
5.3.5 Data acquisition and storage .23
5.3.6 Data quality control and correction .23
5.4 Pyranometers measuring reflected radiation .24
5.4.1 General.24
5.4.2 Installation .24
5.4.3 Inspection and maintenance .25
5.4.4 Data acquisition and storage .25
5.4.5 Data quality control and correction .25
5.5 Pyranometer calibration and performance verification .26
5.5.1 Calibration .26
5.5.2 On-site performance verification/check .28
5.5.3 Introduction of a new pyranometer sensitivity .28
5.6 Uncertainty evaluation of the measurement .29
5.7 Indoor use of pyranometers .32
Annex A (informative) Heating and ventilation systems .34
Annex B (informative) Shading losses in reflected radiation measurement .36
Bibliography .38
© ISO 2021 – All rights reserved PROOF/ÉPREUVE iii

---------------------- Page: 3 ----------------------
ISO/TR 9901:2021(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO’s adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 180, Solar energy, Subcommittee SC 1,
Climate — Measurement and data.
This second edition cancels and replaces the first edition (ISO/TR 9901:1990), which has been
technically revised.
The main changes compared to the previous edition are as follows:
— adaptation of the terminology to the revised ISO 9060:2018 including reference to new “non
spectrally flat” and “fast response” instruments;
— added recommended practices for use of modern pyranometers with a digital output, including
internal diagnostics;
— added recommended practices for use of pyranometers to measure “plane of array” and reflected
radiation;
— added references to the main standards used in solar energy application of pyranometers:
IEC 61724-1:2017, ASTM G213-17 and ASTM G183-15.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
iv PROOF/ÉPREUVE © ISO 2021 – All rights reserved

---------------------- Page: 4 ----------------------
ISO/TR 9901:2021(E)

Introduction
This document contains recommendations for use of pyranometers in solar energy applications. It
summarises the state of the art and updates the first edition of 1990. In recent years the application
of solar radiation measurement, using pyranometers, has risen sharply. The main application of
pyranometers now is no longer scientific research, but assessment of the performance of PV solar
power plants, that is power plants employing photovoltaic solar modules. The reflected irradiance
measurement also has become more relevant with the increasing application of bifacial modules.
Between 1990 and now the use of pyranometers has been further standardized. Two examples are
the 2017 revision of IEC 61724, the group of standards governing use of PV system performance
monitoring, and the 2018 revision of ISO 9060 covering pyranometer and pyrheliometer specification
and classification. The IEC standard implicitly recognises that solar irradiance is a critical and often
the least accurately known parameter in solar energy performance assessment. For those users that
choose to work according to this standard, IEC 61724-1 now defines 3 monitoring system classes and
offers detailed guidelines for use of pyranometers including requirements (not recommendations) for
the pyranometer classes that must be used, for instrument heating and for inspection-, cleaning and re-
calibration intervals.
The solar community also has come to realise that a measurement without an uncertainty evaluation
is meaningless. IEC 61724-1 requires this evaluation when measurement results are reported, usually
as PV performance ratio and performance index. ASTM has issued the G213 standard in 2017 for
uncertainty evaluation of the measurement with pyranometers.
The 1990 version of ISO TR 9901 included reference only to “spectrally flat” pyranometers. Now
that ISO 9060 in its latest version also defines and classifies “non spectrally flat” pyranometers, this
document also refers to the use of these instruments.
As in all above documents, uncertainties mentioned in this document are expanded uncertainties with
a coverage factor k = 2.
© ISO 2021 – All rights reserved PROOF/ÉPREUVE v

---------------------- Page: 5 ----------------------
TECHNICAL REPORT ISO/TR 9901:2021(E)
Solar energy — Pyranometers — Recommended practice
for use
1 Scope
This document gives recommended practice for the use of pyranometers in solar energy applications
(e.g. testing of solar photovoltaic panels, solar thermal collectors or other devices, and performance
monitoring of solar energy systems). It is applicable for both outdoor and indoor use of pyranometers,
when measuring plane of array, global horizontal and reflected irradiance, or radiation from a solar
simulator. The measurement may be carried out on either a horizontal or an inclined surface, and the
pyranometer may be part of a diffusometer, i.e. combined with a sun-shading device to measure diffuse
radiation.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, following the terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
pyranometer
radiometer designed for measuring the irradiance on a plane receiver surface which results from the
radiant fluxes incident from the hemisphere above within the wavelength range from approximately
0,3 µm to about 3 µm to 4 µm
[SOURCE: ISO 9060:2018, 3.5, modified — Note 1 to entry was deleted.]
3.2
hemispherical radiation
solar radiation received by a plane surface from a solid angle of 2π sr
[SOURCE: ISO 9060:2018, 3.1, modified — Note 1 to entry was deleted.]
3.3
global horizontal irradiance
GHI
hemispherical radiation (3.2) received by a horizontal plane surface, also denoted as G
[SOURCE: ISO 9060:2018, 3.2, modified — "GHI" was added as abbreviated term and "also denoted as G"
was added at the end of the definition.]
© ISO 2021 – All rights reserved PROOF/ÉPREUVE 1

---------------------- Page: 6 ----------------------
ISO/TR 9901:2021(E)

3.4
direct radiation
radiation received from a small solid angle centred on the sun’s disc, on a given plane
Note 1 to entry: Reference [3] recommends an opening half angle of 2.5° and a slope angle of 1°. In general, direct
radiation is measured by instruments with field-of-view angles of up to 6°. Therefore, a part of the scattered
radiation around the sun’s disc (circumsolar radiation or aureole) is also included (see 5.1).
Note 2 to entry: Approximately 97 % to 99 % of the direct radiation received at the ground is contained within
the wavelength range from 0,3 μm to 3 μm.
[SOURCE: ISO 9060:2018, 3.3, modified — "solar" was deleted from the term, Note 1 to entry was
modified and Note 3 to entry was deleted.]
3.5
direct normal irradiance
DNI
radiation received from a small solid angle centred on the sun’s disc, on a plane normal to its direction
3.6
diffuse radiation
hemispherical radiation (3.2) minus coplanar direct radiation (3.4)
Note 1 to entry: For the purposes of solar energy technology, diffuse radiation includes solar radiation scattered
in the atmosphere as well as solar radiation reflected by the ground, depending on the inclination of the receiver
surface.
[SOURCE: ISO 9060:2018, 3.4, modified — Note 2 to entry was deleted.]
3.7
diffuse horizontal irradiance
DHI
global horizontal irradiance (3.3) minus coplanar direct (the portion emanating from the solar disk and
from the circumsolar region of the sky within a subtended full angle of 5°).
[SOURCE: IEC 61724-1:2017]
3.8
plane of array irradiance
POA
sum of direct, diffuse, and ground-reflected irradiance incident upon the frontside of an inclined surface
parallel to the plane of the modules in the PV array
[SOURCE: IEC 61724-1:2017]
3.9
reflected irradiance
RI
ground-reflected irradiance incident upon a defined surface, typically parallel to the plane of the
modules in the (bifacial) PV array
[SOURCE: IEC 61724-1:2017]
3.10
rearside plane of array irradiance
REAR
POA
is the sum of direct, diffuse, and ground-reflected irradiance incident on the back side of an inclined
surface parallel to the plane of the modules in the PV array
[SOURCE: IEC 61724-1:2017]
2 PROOF/ÉPREUVE © ISO 2021 – All rights reserved

---------------------- Page: 7 ----------------------
ISO/TR 9901:2021(E)

3.11
reflected horizontal irradiance
RHI
ground-reflected irradiance incident upon a surface, oriented horizontally facing down
[SOURCE: IEC 61724-1:2017]
3.12
accuracy class
class of measuring instruments or measuring systems that meet stated metrological requirements that
are intended to keep measurement errors or instrumental uncertainties within specified limits under
specified operating conditions
[SOURCE: JCGM 200:2012]
3.13
sample
data acquired from a sensor or measuring device
[SOURCE: IEC 61724-1:2017]
3.14
sampling interval
time between samples
[SOURCE: IEC 61724-1:2017]
3.15
record
data recorded and stored in data log, based on acquired samples
[SOURCE: IEC 61724-1:2017]
3.16
recording interval
time between records
[SOURCE: IEC 61724-1:2017]
3.17
clearness index
k
ratio of the GHI to the irradiance that would be available without the earth’s atmosphere (i.e. the GHI
divided by the extra-terrestrial irradiance received at the same sun incidence angle, k = G/G )
o
Note 1 to entry: The extra-terrestrial irradiance at normal incidence used for calculation of the clearness index
[17]
is the Solar constant (1361,1 W/m²) corrected by a sinusoidal variation of amplitude 3,3 % to account for the
sun-earth distance variation over the year. The clearness index may be considered as an attenuation factor of the
atmosphere or the atmospheric transmittance.
3.18
reference operating condition
reference condition
operating condition prescribed for evaluating the performance of a measuring instrument or measuring
system or for comparison of measurement results
Note 1 to entry: For practical purposes these are often the conditions for which the calibration is valid.
[SOURCE: JCGM 200:2012]
© ISO 2021 – All rights reserved PROOF/ÉPREUVE 3

---------------------- Page: 8 ----------------------
ISO/TR 9901:2021(E)

3.19
calibration of a pyranometer
determination of the instrument sensitivity, under well-defined reference operating conditions (3.18)
Note 1 to entry: See also ISO 9846.
4 Selection of pyranometers and accessories
4.1 General
2
A pyranometer performs a hemispherical irradiance measurement in W/m . It is important to realise
[21]
that in many applications for example when working according to monitoring standards IEC 61724-1 ,
[22] [3]
ASTM G183 and WMO a measurement are accompanied by a time stamp. Both the irradiance and
the time stamp have a measurement uncertainty. See 5.2.5.2 and 5.6 for more details on uncertainty
evaluation.
The pyranometer selection is often based on the wish to attain a certain measurement uncertainty.
There also may be other considerations:
a) Task-specific criteria, such a maximum response time, or the requirement to comply with a
standard.
b) Operational criteria, such as dimensions, weight, stability, measures to mitigate dew, frost,
precipitation and soiling, and maintenance requirements of the instrument and accessories.
c) Economic criteria, costs of mechanical and electrical integration in a system depend on the
instrument characteristics. Also costs of recalibration, inspection and maintenance may be
considered.
When selecting an instrument there are two common ways to make a choice, described in the following
clauses:
— related to the pyranometer accuracy class;
— related to the specifications of the pyranometer and its accessories.
4.2 Pyranometer selection based on accuracy class
In some applications the choice of instrument is driven by the pyranometer accuracy class. The class
is often, but not necessarily related to the type, i.e. the technology used (e.g. with photodiode or
thermopile sensors).
The choice of a certain accuracy class is often driven by the requirements of standards. Table 1
summarizes the required pyranometer accuracy class for the most common application of PV system
performance monitoring according to IEC.
NOTE IEC 61724-1 is due for revision in 2021, and requirements will possibly change.
ISO 9060:2018 defines 3 pyranometer classes, A, B and C. These classes are “accuracy classes”, which
are defined by JCGM200: 2012 to meet stated metrological requirements that are intended to keep
measurement errors or instrumental uncertainties within specified limits under specified operating
[24]
conditions .
The accuracy classification as used in ISO 9060 does not by definition mean that a higher class
pyranometer will provide a higher accuracy measurement; this entirely depends on the application.
Besides classification as class A, B, and C, ISO 9060 makes a further distinction between 2 main types
and an independent sub-category:
— spectrally flat pyranometers; most thermoelectric pyranometers are in this category;
4 PROOF/ÉPREUVE © ISO 2021 – All rights reserved

---------------------- Page: 9 ----------------------
ISO/TR 9901:2021(E)

— (non spectrally flat) pyranometers; photodiode pyranometers may qualify for this category;
a further sub-category of fast-response pyranometers.
Table 1 — Application of pyranometers of different ISO 9060 accuracy classes
for the most common solar energy studies
Application for solar energy studies ISO 9060 instrument accuracy class and comments
IEC 61724-1 PV system performance moni- ISO 9060 spectrally flat class A, with dew and frost mitigation in
toring class A case these have a significant impact on the measurement accura-
cy
IEC 61724-1 PV system performance moni- ISO 9060 spectrally flat class B, with dew and frost mitigation in
toring class B case these have a significant impact on the measurement accura-
cy
NOTE: IEC 61724-1 is expected to be updated in 2021, and requirements of a new version of the standard may be different
from those stated in above table.
Pyranometers classified in ISO 9060 as “spectrally flat” have a spectral selectivity of less than 3 %
(guard bands 2 %) in the 0.35 to 1.5 µm spectral range. This is the same requirement as in the previous
ISO 9060:1990 for secondary standard pyranometers. Spectrally flat pyranometers are typically
more accurate over a wide range of conditions, and applicable not only for horizontal measurement
of global horizontal irradiance, GHI, but also for measurements of plane of array irradiance, POA, and
reflected irradiance, RI, as well as for artificial solar sources such as lamps. IEC 61724-1 requires use of
instruments of a specified accuracy class for its class A and B monitoring systems. There is consensus
that ISO 9060 “Spectrally flat” pyranometers’ spectral selectivity specifications have a negligible (zero)
spectral error and that they can be used for all the common outdoor measurements in solar energy
studies with the same calibration (typically performed with the clear sky solar spectrum as the source)
without significant loss of accuracy. The clear sky solar spectrum is one of the reference operating
conditions for pyranometers if it is the source under which an instrument is calibrated or the source
under which a calibration reference standard has been calibrated.
Pyranometers employing photodiodes (otherwise known as silicon-pyranometers), are not classified as
“spectrally flat” in ISO 9060. The spectral error of pyranometers is defined for a set of clear sky solar
spectra only. This implies that their spectral error for other than clear sky spectra cannot be based
on the classification alone. The spectral error of pyranometers, in particular if they are not spectrally
flat, may be larger for measurements of DHI, POA or RI than for clear sky GHI. The user may perform
an individual uncertainty evaluation depending on the manufacturer specification of the instrument
and the spectra of the measured radiation. The factory calibration of non spectrally flat instruments is
typically valid for a set of clear sky solar spectra. Their sensitivity and uncertainty of their sensitivity
may both change for different conditions.
Non spectrally flat pyranometers also may offer specific advantages; they generally are inexpensive,
small and have a fast response time. They may be used for example for temporally highly resolved
measurements, when overall accuracy requirements are not too high, or where constant spectrum
conditions exist (for example, working with artificial sources, or only working under clear sky
conditions). They also may be used for high-accuracy applications when calibrated under the working
conditions.
In summary, spectrally flat pyranometers can be used for the most common solar testing applications,
including GHI, POA, RI and albedo measurements using traceability to the same clear sky spectrum
calibration. When using non spectrally flat pyranometers for other than clear-sky GHI measurements,
the spectral error may be larger than the spectral error specified in ISO 9060.
If a higher measurement accuracy is required than may be attained with a class A pyranometer. There
also are class A pyranometers with improved directional error- and zero-offset specifications.
For the highest accurate measurement it is recommended to derive the hemispherical radiation from
the combined measurements of a pyrheliometer and a shaded (i.e. shielded from direct radiation)
© ISO 2021 – All rights reserved PROOF/ÉPREUVE 5

---------------------- Page: 10 ----------------------
ISO/TR 9901:2021(E)

pyranometer. These measure direct radiation and diffuse radiation respectively. There is international
[8]
consensus that this type of measuring system provides the most accurate measurement possible .
Fast response pyranometers or spectrally flat fast response pyranometers are used when a fast
response leads to a higher measurement accuracy. This may be to study highly variable sky conditions
or over-irradiance events. ISO 9060 requires a 95 % response time <0,5 s to qualify for this sub-
category.
4.3 Pyranometer and accessory selection based on other considerations
The accuracy classification of ISO 9060 does not by definition mean that a higher class pyranometer
will provide a higher accuracy measurement; this entirely depends on the application. Users need to
consider the suitability of a pyranometer not only based on the type or accuracy class, but also based on
the detailed specifications of the pyranometer and its accessories.
As a first step, the requirements for the spectral response, see 4.2, and the operating conditions
(temperature, irradiance, angle of incidence, tilt angle) may be established. The range of irradiance and
ranges of operating conditions in indoor tests are usually smaller than those in outdoor tests, see 5.7
for indoor testing.
As a second step users could look at the accessories.
Reference is usually made to measuring- and other specifications such as:
— specifications possibly exceeding those necessary for ISO 9060 classification, such as low zero
offsets, good directional response, extended spectral range, faster response time, extended
temperature range, as given by the manufacturer, or as established by testing;
— specifications of accessories such as external ventilations systems, shading mechanisms, etc.;
— additional measu
...