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Operated by the Alliance for Sustainable Energy, LLC

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Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

Conference Paper 

NREL/CP-5D00-75879 

July 2020 

Consensus International Solar 

Resource Standards and Best Practices 

Development

Preprint

Aron Habte,

1 Manajit Sengupta,1 and 

Christian A. Gueymard2 

1 National Renewable Energy Laboratory

2 Solar Consulting Services 

Presented at the 47th IEEE Photovoltaic Specialists Conference (PVSC 47)

June 15–August 21, 2020 
NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

Conference Paper

NREL/CP-5D00-75879 

July 2020 

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

303-275-3000 • www.nrel.gov

Consensus International Solar 

Resource Standards and Best Practices 

Development

Preprint

Aron Habte,1 Manajit Sengupta,1 and 

Christian A. Gueymard2 

1 National Renewable Energy Laboratory

2 Solar Consulting Services 

Suggested Citation 

Habte, Aron, Manajit Sengupta, and Christian A. Gueymard. 2020. Consensus 

International Solar Resource Standards and Best Practices Development: Preprint. 

Golden, CO: National Renewable Energy Laboratory. 

NREL/CP-5D00-75879. https://www.nrel.gov/docs/fy20osti/75879.pdf. 

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1 

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Consensus International Solar Resource Standards 

and Best Practices Development

Abstract—Standardization and best practices of data sets 

and models enable the industry to develop widely accepted protocols adapted to various stages of solar project development 

and operations. In collaboration with the International Energy 

Agency Photovoltaic Power Systems Programme Task 16, the 

National Renewable Energy Laboratory (NREL) developed and 

regularly updates the Best Practices Handbook for the Collection 

and Use of Solar Resource Data for Solar Energy Applications. In 

collaboration with ASTM’s radiometry subcommittee, NREL 

leads the development of various radiometric standards. These

contribute to increased accuracy in the measurement of the solar resource available to conversion systems and boost the 

development and improvement of radiative models. These models can then provide the long-term geographic or climatic

distribution of solar radiation for areas where measured solar 

data are not available. Further, these standards play a preeminent role in all aspects of solar energy projects, including 

standard conditions, methods and instrumentation, accelerated 

testing, and service lifetime of materials. 

Keywords—ASTM, Best Practices, Standards, Solar Resource, 

Solar Radiation

I. INTRODUCTION

A lack of common terminology, common methods, and/or 

awareness of national and international best practices can lead 

to over- or underestimates of the available solar resource. In 

turn, this typically induces incorrect estimates of the long-term 

performance of a given solar power facility and consequently

results in over- or undersized equipment, bankability risks, 

increased financing costs, or poor financial performance of 

any projected solar power plant. 

The National Renewable Energy Laboratory (NREL) has 

been actively leading and participating in the International 

Energy Agency (IEA) Photovoltaic Power Systems (PVPS) 

Programme Task 16 and ASTM subcommittee G03.09 on 

radiometry standards development. Standards and best 

practices related to the solar resource contribute to a better 

understanding of the magnitude and uncertainty of the 

resource available to conversion systems. Such documents

also contribute to the development and improvement of solar 

resource models for areas where solar radiation measurements

are not available. Moreover, the service lifetime of materials 

used in solar conversion systems—including polymer 

encapsulants, mirrors, and coatings evaluation—requires solar 

resource information that is based on known traceable 

standards and gathered appropriately by following the proper 

best practices to maintain data quality [1]. Most importantly, 

the performance of photovoltaic (PV) cells, modules, and 

arrays that form large solar deployments rely on accurate 

measurements of the available solar resource. Determining the 

accuracy of the solar resource acquired through measurement

or models therefore provides a better understanding of 

investment risks. This becomes especially important as the 

deployment sizes increase and investment costs reach many

millions of dollars. Further, these standards play a preeminent 

role in all aspects of weathering and durability, including 

standard conditions, methods and instrumentation, accelerated 

testing, and service lifetime of materials systems. Because 

solar conversion systems are deployed outdoors, the 

performance over time of component materials of systems is 

important. Many system components are tested using 

accelerated weathering systems in the laboratory or natural 

exposure techniques outdoors. Overall, the standards and best 

practices being developed or updated support the present and 

future deployment of all solar conversion systems. 

II. STANDARDS ACTIVITIES

The development of consensus standards and handbooks 

for solar resource modeling and measurement includes various topics: (i) calibration, measurement, application, and data 

processing; (ii) developing and providing guidelines or standard procedures for establishing data uncertainties; (iii) 

developing and validating test procedures for optical radiation applications; (iv) undertaking best practices 

measurement activities at selected high-value sites around the 

United States for benchmarking; and (v) understanding resource variability, solar resource modeling, and forecasting 

methodologies. All this constitutes essential information for 

various solar energy conversion stakeholders (Fig. 1).

NREL leads the ASTM G03.09 radiometry subcommittee, which is under ASTM Committee G03: Weathering and 

Durability. The subcommittee develops and maintains standards associated with the specification, calibration, 

measurement, and modeling of solar radiation. The latter encompasses the ultraviolet (UV), visible, and infrared 

radiation regions, regardless of the radiation source. 

NREL is also a U.S. Technical Advisory Group in the International Organization for Standardization (ISO)/Technical 

Committee (TC) 180/SC 1 Climate - Measurement and Data. 

The committee promotes knowledge and stimulates research 

Aron Habte and Manajit Sengupta

National Renewable Energy Laboratory 

Golden, Colorado 

Aron.Habte@nrel.gov; Manajit.Sengupta@nrel.gov

Christian A. Gueymard

Solar Consulting Services

P.O. Box 392

Colebrook, NH

Chris@SolarConsultingServices.com
2 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications.

on the calibration and specification of radiometers, the development of reference spectral irradiance, and radiometers’ 

recommended practice for use.

Fig.1. Standards and best practices activities.

III. ASTM SUBCOMMITTEE G03.09: CURRENT STANDARDS

A. ASTM G213-17: Standard Guide for Evaluating 

Uncertainty in Calibration and Field Measurements of 

Broadband Irradiance with Pyranometers and 

Pyrheliometers

As stated in ASTM G213-17, this standard prescribes

methods to evaluate uncertainties resulting from the calibration of pyranometers or pyrheliometers, or from field 

measurements made by them [2]. Moreover, the standard 

aims to maintain the measurement traceability with respect to 

the International System of Units, which ensures that the uncertainty quoted for radiometric measurements can be 

compared according to documented methods of derivation. 

This standard is relevant for solar conversion systems, 

such as PV and concentrating solar power (CSP). To evaluate

the performance of PV or CSP plants, accurate measurements 

of the available solar resource with known uncertainty are 

necessary for design purposes, for a better understanding of 

investment risks and for meeting contractual obligations. As

the deployment size of these technologies increases and investment costs reach hundreds of millions of dollars for large 

utility-scale power plants, characterizing the accuracy of 

measurements becomes essential. Therefore, having traceable methods for estimating the measurement uncertainty of 

the solar resource assists in reducing risks in project design

and financing, shortening deployment times, and enhancing 

system operational efficiency.

B. ASTM G214-16: Test Method for Integration of Digital 

Spectral Data for Weathering and Durability 

Applications

This standard discusses and recommends the modified 

trapezoid rule, a simple integration method frequently used to 

compute the total integrated irradiance over a given wavelength interval [3]. The standard describes a method to derive

uniform and consistent integrated digital or tabulated spectral 

data for various applications. Among them, PV characterization requires the short-circuit current, which is estimated by 

integrating both the incident spectral irradiance and the cell’s

spectral response [4]. Another application is the comparison 

of calibration results from narrow- and broadband UV radiometers that are calibrated using spectroradiometers [5]; see 

subsection G below. 

C. ASTM E816-15 Standard Test Method for Calibration of 

Pyrheliometers by Comparison to Reference 

Pyrheliometers

This standard describes two methods to transfer calibration of pyrheliometers: (i) The calibration of a secondary 

reference pyrheliometer using an absolute cavity radiometer

as the primary standard instrument (e.g., NREL’s Broadband 

Outdoor Radiometer Calibration [BORCAL] method). According to this standard, the outdoor exposure of these two 

instruments shall be limited to calibration or intercomparison 

events only; and (ii) The transfer of calibration from a secondary reference to one or more field pyrheliometers[6]. This 

test method recommends the calibration chain and traceability to the World Radiometric Reference (WRR). This 

standard is technically comparable to and consistent with ISO 

9059 [7]. 

D. ASTM G167-15 Standard Test Method for Calibration of 

a Pyranometer Using a Pyrheliometer

This standard [8] details two types of calibration methods: 

one involves a self-calibrating, absolute cavity radiometer, 

which is similar to NREL’s BORCAL method (Fig. 2), 

whereas the other uses a secondary reference pyrheliometer 

as the reference standard as defined by the World Meteorological Organization and ISO 9060 [9]. 

Fig. 2. NREL’s BORCAL method, which involves a self-calibrating, absolute cavity radiometer (left) and a diffuse pyranometer on a tracker 

(right). Photos by NREL
3 

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E. ASTM E824-10 (2018)e1 Standard Test Method for 

Transfer of Calibration from Reference to Field 

Radiometers

This standard describes a step-by-step method for the 

transfer to field radiometers of the calibration from reference 

radiometers that are traceable to the WRR [10]. The reference 

radiometers are calibrated using ASTM G167. The method 

described in ASTM E824 applies to radiometers, such as pyranometers, that measure hemispherical radiation incident on 

a flat surface. 

F. ASTM G138-12 Standard Test Method for Calibration of 

a Spectroradiometer Using a Standard Source of 

Irradiance

In this standard, reference lamps that are traceable to national metrological institutes (NMIs) are used to calibrate

spectroradiometers for the measurement of spectral irradiance. According to this standard, each NMI shall participate 

in intercomparisons of the spectral irradiance standards [11]. 

NREL’s ISO 17025 spectral calibration accreditation depends on this standard (Fig. 3). 

Fig. 3. NREL’s spectroradiometer calibration setup traceable to the National Institute of Standards and Technology: a tungsten-filament, 1,000-

W quartz-halogen FEL (top), and a spectroradiometer (bottom) are used 

to calibrate a solar simulator. Photos by NREL

G. ASTM G130-12 Standard Test Method for Calibration of 

Narrow- and Broad-Band Ultraviolet Radiometers 

Using a Spectroradiometer

This standard prescribes a scanning or linear-diode-array 

spectroradiometer calibrated using ASTM G138 to calibrate 

narrow- or broadband UV radiometers [5]. The calibration 

accuracy resulting from the use of this standard depends on 

the accuracy of the light source and calibration setup; therefore, care should be taken to minimize the errors during 

calibration. The Solar Radiation Research Laboratory at 

NREL performs UV calibrations using this standard.

H. ASTM G173-03(2012) Standard Tables for Reference 

Solar Spectral Irradiances: Direct Normal and 

Hemispherical on 37° Tilted Surface

This standard is widely used by the solar community to 

generate spectral clear-sky reference data for use in understanding absorptance, reflectance, and transmittance of solar 

energy materials[12], which are important factors in solar PV 

system performance, material reliability studies, solar thermal system performance, and solar simulation activities. The 

direct normal irradiance spectrum is typically applied to the 

determination of spectral matching ratios [13]. The two spectra are for an air mass of 1.5 and form the basis of the 

International Electrotechnical Commission standard used for 

the rating of flat-plate PV devices and concentrating PV terrestrial solar cells. Subordinate standards—derived for 

different atmospheric conditions than those selected to establish G173—have also been proposed [14], and they are under 

consideration by ISO.

I. ASTM G177-03(2012) Standard Tables for Reference 

Solar Ultraviolet Spectral Distributions: Hemispherical 

on 37° Tilted Surface

Similar to ASTM G173, this standard contains a reference

spectral irradiance, but limited here to UV wavelengths 

(0.28‒0.40 μm). The specified spectral irradiance distribution 

has many applications, such as in exposure testing where light 

sources are evaluated against this reference [15]. This UV 

spectrum is for a low air mass (1.05) and high atmospheric 

transparency; hence, by design, its magnitude is significantly 

larger than that of the global spectrum in G173 (Fig. 4). 

Fig. 4. Global irradiance spectral distribution in the UV according to 

ASTM G173 and G177.
4 

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J. ASTM G183-15 Standard Practice for Field Use of 

Pyranometers, Pyrheliometers and UV Radiometers

This standard describes best practices of station commissioning conditions, periodical upkeep requirements, 

operations feedback, and calibration intervals for use of pyranometers, pyrheliometers, and UV radiometers in outdoor 

testing environments [16] (Fig. 5). This best practice also discusses the conditions that prescribe the level of accuracy 

required for various radiometer types. 

Fig. 5. Best practices sequence according to ASTM G183.

K. ASTM G197-14 Standard Table for Reference Solar 

Spectral Distributions: Direct and Diffuse on 20° Tilted 

and Vertical Surfaces 

This standard provides solar spectral irradiance distributions that are necessary to develop spectral weighting 

functions of the optical properties of fenestration systems or 

the performance of PV device technologies in buildings [17]. 

This standard is used by the National Fenestration Rating 

Council, for instance. The impact of receiver tilt (20° and 90° 

for G197, vs. 37° for G173) on spectral distribution is shown 

in Fig. 6. 

Fig. 6. Global irradiance spectral distribution on various tilts according to 

ASTM G173 and G197.

L. ASTM G207-11(2019)e1 Standard Test Method for 

Indoor Transfer of Calibration from Reference to Field 

Pyranometers

The method described in this standard is similar to ASTM 

E824 except that standard G207 is pertinent to the indoor 

transfer of calibration from reference to field radiometers. 

The radiometers calibrated using this standard are used for 

various applications, such as for measuring and monitoring 

outdoor radiant exposure levels [18], while avoiding the difficulties and time requirements of outdoor calibration. Most 

radiometer manufacturers depend on this standard.

M. ISO 9060:2018 Solar energy – Specification and 

classification of instruments for measuring

The ISO/TC 180/SC 1 subcommittee recently updated 

ISO 9060:2018. That standard describes the available international specifications for the performance classification for 

instruments that measure irradiance [9]. The changes contained in the update include new classes of sensors as well as

notes on spectral selectivity, spectral error calculation, application of correction function, and discussion about shading 

devices.

N. Update of Solar Resource for Solar Energy Applications 

Handbook

In collaboration with international partners within the 

IEA-PVPS Task 16, NREL is leading the revision of the Best 

Practices Handbook for the Collection and Use of Solar Resource Data for Solar Energy Applications: Second Edition

[19]. The third edition of this widely used Handbook, slated 

for publication at the end of 2020, will include current progress of solar resource measurement and modeling, progress 

in solar forecasting, and expert advice on various solar resource aspects of solar energy applications.

IV. CONCLUSION

NREL is involved in the development and update of 

standards and best practices relevant to solar energy applications. This effort supports the development of international 

consensus standards in solar measurement and modeling to 

represent the state-of-the-art knowledge through continuous 

formal engagement of various stakeholders. Standardization 

and best practices of data sets and models enables the industry 

to develop fair and widely accepted protocols for various 

stages of solar energy project development and operations, 

strengthen consumer confidence, and assist in refining the efficiency and durability of their products. This also reduces 

barriers to seeking and successfully obtaining financing and 

reduces warranty costs for solar energy projects. 

To reinforce these efforts, NREL, national and international organizations, and international standards bodies 

continuously seek support to increase participation and develop new capabilities within the committees.

ACKNOWLEDGMENTS

This work was authored in part by Alliance for Sustainable Energy, LLC, the manager and operator of the National 

Renewable Energy Laboratory for the U.S. Department of 

Energy (DOE) under Contract No. DE-AC36-08GO28308. 
5 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications.

Funding provided by the U.S. Department of Energy Office 

of Energy Efficiency and Renewable Energy Solar Energy 

Technologies Office. The views expressed in the article do 

not necessarily represent the views of the DOE or the U.S. 

Government. The U.S. Government retains and the publisher, 

by accepting the article for publication, acknowledges that 

the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the 

published form of this work, or allow others to do so, for U.S. 

Government purposes. 

We are grateful to the U.S. Department of Energy Solar 

Energy Technologies Office and the systems integration subprograms for supporting this project. Specifically, we 

acknowledge Dr. Tassos Golnas and Dr. Guohui Yuan for 

their support and encouragement.

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[2] ASTM G213-17, “Standard Guide for Evaluating Uncertainty in 

Calibration and Field Measurements of Broadband Irradiance with 

Pyranometers and Pyrheliometers,” ASTM International, West 

Conshohocken, PA, 2017, www.astm.org. 

[3] ASTM G214-16, “Standard Test Method for Integration of Digital 

Spectral Data for Weathering and Durability Applications,” ASTM 

International, West Conshohocken, PA, 2016, www.astm.org. 

[4] R. Hulstrom, R. Bird, and C. Riordan, “Spectral Solar Irradiance Data 

Sets for Selected Terrestrial Corrections,” Solar Cells, vol. 15, pp 365-

391, 1985.

[5] ASTM G130-12, “Standard Test Method for Calibration of Narrowand Broad-Band Ultraviolet Radiometers Using a Spectroradiometer,” 

ASTM International, West Conshohocken, PA, 2012, www.astm.org.

[6] ASTM E816-15, “Standard Test Method for Calibration of 

Pyrheliometers by Comparison to Reference Pyrheliometers,” ASTM 

International, West Conshohocken, PA, 2015, www.astm.org. 

[7] ISO 9059:1990, “Solar energy - Calibration of field pyrheliometers by 

comparison to a reference pyrheliometer”. International Standard.

[8] ASTM G167-15, “Standard Test Method for Calibration of a 

Pyranometer Using a Pyrheliometer,” ASTM International, West 

Conshohocken, PA, 2015, www.astm.org.

[9] ISO 9060:2018. 2018, “Solar energy — Specification and classification 

of instruments for measuring hemispherical solar and direct solar 

radiation,” International Standard.

[10] ASTM E824-10(2018)e1, “Standard Test Method for Transfer of 

Calibration From Reference to Field Radiometers,” ASTM 

International, West Conshohocken, PA, 2018, www.astm.org.

[11] ASTM G138-12, “Standard Test Method for Calibration of a 

Spectroradiometer Using a Standard Source of Irradiance,” ASTM 

International, West Conshohocken, PA, 2012, www.astm.org.

[12] ASTM G173-03(2012), “Standard Tables for Reference Solar Spectral 

Irradiances: Direct Normal and Hemispherical on 37° Tilted Surface,” 

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[13] R. Núñez, C. Domínguez, S. Askins, M. Victoria, R. Herrero, I. Antón, 

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[15] ASTM G177-03(2012), “Standard Tables for Reference Solar 

Ultraviolet Spectral Distributions: Hemispherical on 37° Tilted 

Surface,” ASTM International, West Conshohocken, PA, 2012, 

www.astm.org. 

[16] ASTM G183-15, “Standard Practice for Field Use of Pyranometers, 

Pyrheliometers and UV Radiometers,” ASTM International, West 

Conshohocken, PA, 2015, www.astm.org. 

[17] ASTM G197-14, “Standard Table for Reference Solar Spectral 

Distributions: Direct and Diffuse on 20° Tilted and Vertical Surfaces,” 

ASTM International, West Conshohocken, PA, 2014, www.astm.org

[18] ASTM G207-11(2019)e1, “Standard Test Method for Indoor Transfer 

of Calibration from Reference to Field Pyranometers,” ASTM 

International, West Conshohocken, PA, 2019, www.astm.org

[19] M. Sengupta, A. Habte, C. Gueymard, S. Wilbert, D. Renne, and T. 

Stoffel, “Best Practices Handbook for the Collection and Use of Solar 

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