NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Monitoring System Performance

Venue: PV Module 

Reliability Workshop

Presenter: Keith Emery 

and Ryan Smith

Date: February 16, 2011

NREL/PR-5200-50643
NATIONAL RENEWABLE ENERGY LABORATORY

Outline

Standards

– National Electric Code

– IEC 60904-1 “Photovoltaic devices – Part 1: Measurement of 

photovoltaic current-voltage characteristics”

– IEC 61724 “Photovoltaic System Performance Monitoring – Guidelines for Measurement, Data Exchange and Analysis”

– IEC 61829 “Crystalline Silicon Photovoltaic array – On-site 

Measurement of I-V Characteristics”

– ASTM E1036 “Standard Test Methods for Electrical Performance of 

Nonconcentrator Terrestrial Photovoltaic Modules and Arrays Using 

Reference Cells”

– ASTM E2527 “Standard Test Method for Electrical Performance of 

Concentrator Terrestrial Photovoltaic Modules and Systems Under 

Natural Sunlight”

– ASTM WK22009 “Reporting Photovoltaic Non-Concentrator System 

Performance “

Sources of Uncertainty

NREL’s Approach to Local and Remote System Monitoring

Summary

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• Voltage and Current measured with ±0.2 % Uncertainty in 

Voc and Isc

• Test and Reference device coplanar within ±2° and 

normal to the sun within ±5°.

• Reference cell and test device temperature measured with 

±1 °C uncertainty. 

• Optionally test device temperature determined via 

equivalent cell temperature method (IEC 60904-5)

• Spectral mismatch error correction if matched reference cell 

not used (IEC 60904-7). 

• If reference cell >2 °C from calibration temperature 

corrections applied.

• Use IEC 60891 to translate for temperature and irradiance

IEC 60904-1 “Photovoltaic devices – Part 1: Measurement of Photovoltaic 

Current-Voltage Characteristics
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• Voltage and Current measured with ±0.1 % Uncertainty and 

0.05% Resolution for Voc and Isc.

• Test and Reference device coplanar within ±2° and normal to 

the sun within ±10° or report incidence angle.

• Reference cell and test device temperature measured with 

±1 °C uncertainty, 0.1 °C resolution.

• Spectral mismatch error correction applied (ASTM E973).

• Temperature and irradiance corrections applied if temperature 

more than 2°C from reference temperature or irradiance > 5% 

from reference irradiance correct using bilinear method. 

Otherwise current corrected to constant irradiance.

• Average Wind speed for 5 min prior to test. 

ASTM E1036 “Standard Test Methods for Electrical for Electrical 

Performance of Nonconcentrating Photovoltaic Modules and Arrays 

Using Reference cells”
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Meteorology – Total irradiance, in the plane of 

the array

– Ambient air temperature

– Wind speed

Photovoltaic array – Output voltage

– Output current

– Output power

– Module temperature

– Mounting / tracker 

characteristics

Load– Load voltage

– Load current

– Load power

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Energy storage

– Operating voltage

– Current to/from storage

– Power to/from storage

Utility grid

– Utility voltage

– Each phase - Current to/from 

utility grid

– Power to/from utility grid

Back-up sources

– Output voltage

– Output current

– Output power

IEC 61724 “Photovoltaic System Performance Monitoring –

Guidelines for measurement data exchange and analysis”
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DC

Disconnect

AC Monitoring

(V, I, P, PF)

AC

Disconnect

UtilityInteractive

Inverter

Data Acquisition System

Meteorology

• Module Temp

• Ambient Temp

• Irradiance

Utility Grid

Network

Meteorology

PV array

Load

DC Monitoring

(V, I)
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Total Irradiance –

– Measured in plane of array

– Uncertainty including including Instrumentation < 5%

– Pyranometer

– Reference cell or cell in Module Package or Module (IEC 60904-2)

Ambient Temperature –

– Representative of Array Location

– Uncertainty including Instrumentation < 1°C

Wind Speed

– Measured at Height and Location Representative of Array

– Uncertainty including Instrumentation < 0.5 m/s for Speeds < 5 m/s, 

and <10 % of the reading for Speeds > 5 m/s

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IEC 61724 “Photovoltaic System Performance Monitoring –

Guidelines for measurement data exchange and analysis”
NATIONAL RENEWABLE ENERGY LABORATORY

Module Temperature –

– Measured on back of 1 or more modules in representative of location

– Uncertainty including Instrumentation < 1°C

– location on module given in IEC 61829 method A (center of back 

surface of module in center of array field)

– Based upon Voc and equivalent Cell temperature (IEC 60904-5)

Voltage and Current –

– ac and or dc

– Uncertainty including Instrumentation < 1% of reading

Power

– DC Calculated based upon instantaneous and not averaged 

readings or directly measured with wattmeter

– AC power accounts for power factor and harmonic distortion

– Uncertainty including Instrumentation < 2%

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IEC 61724 “Photovoltaic System Performance Monitoring –

Guidelines for measurement data exchange and analysis”
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Sampling

– Parameters which vary directly with irradiance shall be sampled with 1 min 

or less interval. Parameters with larger time constants, an arbitrary interval 

may be specified between 1 min and 10 min. Special consideration for 

increasing the sampling frequency shall be given to any parameters which 

may change quickly as a function of system load.

Linearity

– The difference between measured and applied signal < ±1 % of full scale 

value at of 0, 20, 40, 60, 80, and 100 % of full scale. 

Stability

– 100 % full scale dc signal applied for 6 h. Should the fluctuation of the input 

signal exceed ±0.2 %, the results shall be compensated by using a 

voltmeter with an accuracy better than ±0.2 %.

Integration

– Apply expected maximum sensor signal for > 6 h. Signal times interval 

within 1% of expected value. With shorted input integral less than 1% of 

maximum signal applied times interval.

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Appendix A - Data Acquisition - Irradiance, Temperature, Voltage, Current, Power

IEC 61724 “Photovoltaic System Performance Monitoring –

Guidelines for measurement data exchange and analysis”
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Air temperature

– Resolution 0.1 °C, Uncertainty < 1 °C

– 1 to 2 m upwind , > 2m above ground

– See ASTM D6176 “Standard Practice for Measuring Surface 

Atmospheric Temperature with Electrical Resistance Temperature 

Sensors”

Wind speed

– Uncertainty less than 0.5 m/s

– 1 to 2 m upwind , > 2m above ground

Total direct beam irradiance

– ASTM E816, “Standard Test Method for Calibration of Pyrheliometers 

by Comparison to Reference Pyrheliometers”

– Typical uncertainty < 1.5%

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dc or ac Power Rating to 20 °C ambient temperature, 850 

W/m2 total irradiance, 4 m/s wind speed

ASTM E2527 “Standard Test Method for Electrical Performance of Concentrator 

Terrestrial Photovoltaic Modules and Systems Under Natural Sunlight”
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Air temperature

– Resolution 0.1 °C, Uncertainty < 1 °C (ASTM D6176)

Wind speed

– Uncertainty less than 0.5 m/s

– ultrasonic preferred since no dead band for <0.5 m/s

Plane of Array irradiance

– Pyranometer

• ASTM E824 Test Method for Transfer of Calibration From 

Reference to Field Radiometers “

• ASTM G167 Test Method for Calibration of a Pyranometer Using a 

Pyrheliometer

– Matched Reference cell suitable for outdoor use (ASTM E1040)

• secondary calibration according to ASTM E1362

ac power measured with less than 1.5% uncertainty

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dc or ac Power Rating to agreed upon Reporting Conditions defined by 

an Ambient Temperature, Total Irradiance, and Wind Speed

ASTM WK22009 “Standard Test Method for Reporting Photovoltaic NonConcentrator System Performance” 
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PV Irradiance detector

Reference Cells in metal package

– Uncertainty in measuring the total irradiance of ± 1.5% provided that Isc is 

temperature corrected and size is less than 2 by 2 cm 

– Corrects for spectral effects to the reference spectrum. Exact correction if 

relative quantum efficiencies of reference and PV system identical

– Multiple intercomparisons among ISO 17025 accredited calibration labs 

suggest uncertainty less than ±2% for Si reference cells at 25 °C

– Short-circuit temperature coefficient of Si is 500 to 850 ppm/°C depending on 

spectra measured under. In systems the temperature range may exceed 60 °

or a 3% error in irradiance if temperature not corrected for.

– Regression analysis of field data has a lower mean square error (less noisy)

– Time constant comparable to PV

– Temperature can be controlled potentially giving lower error in calibration

Cell in Module package

– Angular response similar to PV in system

– Quantum efficiency closer to module because packaging similar

– Thermal characteristics similar to PV system

– Slightly higher uncertainty than small area cells.

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• Encapsulated

• Temperature sensor attached or other means to correct 

for temperature

• Packaged cell or cell in module package

Reference Cell Packages

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Pyranometer

– Wide range of uncertainties depending on how calibrated (indoors using a 

sphere vs. outdoors, how mounted, and if corrections for temperature and 

angular response applied

– 1.5 % to >5% for WMO first class, potentially >20% for other units.

– “WMO GUIDE TO METEOROLOGICAL INSTRUMENTS AND METHODS OF 

OBSERVATION” 

http://www.WMO.int/pages/prog/www/IMOP/publications/CIMOGuide/CIMO_Guide-7th_Edition-2008.html

– U95 for pyranometer calibration at NREL on a horizontal surface ±3 to 4% 

depending on multiple factors. Often different from indoor based calibrations 

by over 5%.

– Calibrated outdoors with respect to incident spectrum so calibration is not with 

respect to a reference spectrum Error for Si pyranometer not as large an error 

for thermopile based units. 

– Usually calibrated on a horizontal surface. Calibration varies with tilt angle.

– Calibration dependent on cable orientation. Cable should face to north away 

from sun in northern hemisphere.

– Manufacturers often calibrate indoors with integrating sphere against a 

reference pyranometer.

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NREL single day Pyranometer calibrations, 2 degree bins

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http://www.nrel.gov/aim/
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Leveling Error

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How closely must irradiance sensor be aligned to PV array?

What is impact of PV modules in system misaligned with respect to each other?

K.A. Emery, et al, “SERI Results from the PEP 1987 Summit Round Robin and a Comparison of Photovoltaic Calibration Methods,” SERI tech. rep. TR-213-3472, March 1989. 
NATIONAL RENEWABLE ENERGY LABORATORY

Si secondary calibrations

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Cell Calibrated Isc (A) Isc,ECN ΔI (%)

100-A ESTI 2002 3.020 ± 1.9% 3.032 +0.4

100-B ESTI 2002 3.006 ± 1.9% 3.019 +0.4

100-B NREL 2003 3.023 ± 2.0% 3.019 -0.1

125-A ESTI 2002 4.780 ± 1.9% 4.816 +0.8

125-B ESTI 2002 4.731 ± 2.0% 4.716 -0.3

125-C NREL 2004 5.311 ± 2.0% 5.311 ---

125-D NREL 2004 5.380 ± 2.0% 5.393 +0.2

125-E PTB 2004 4.752 ± 2.9% 4.724 -0.6

150-A ESTI 2002 6.858 ± 1.9% 6.808 -0.7

150-B PTB 2004 7.247 ± 2.8% 7.122 -2.2

Arvid van der Heide, Stefan Winter, Keith Emery, and Wim Zaaiman, "Comparison Between Large Reference Cells Calibrated by ESTIJRC, NREL and PTB, Performed at ECN", Proc. 20th EU PVSEC, June, 2005. 

Formal and informal secondary reference cell 

intercomparisons among ISO 17025 accredited or equivalent 

calibration labs suggest that the uncertainty is ±2% or less.
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NREL’s Approach to Local and Remote System Monitoring

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Successful monitoring requires understanding the entire data 

acquisition system.

Budget constraints may dictate to what extent monitoring is possible.

Local and remote monitoring applications can require different 

approaches.

Primary concepts for either monitoring application include:

• Select highly reliable hardware suitable for the end-use environment

• Decrease the susceptibility to external noise

• Choose sensors with low uncertainties and low drift

• In general, a simple system design is better

• Calibrate! Calibrate! Calibrate! - Calibration drift adversely impacts data 

quality and interpretation (next talk).
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Typical NREL Installation

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DC

Disconnect

DC Monitoring

(V, I)

AC Monitoring

(V, I, P, PF)

AC

Disconnect

UtilityInteractive

Inverter

Data Acquisition System

External Sensors

• Module Temp

• Ambient Temp

• Irradiance

Utility Grid

Network
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Choices

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Reliable hardware is worth the cost!

• If the data is to be used for modeling or analysis than it needs to be as accurate and 

complete as possible. Unreliable hardware can impact results because of missing, 

inaccurate, or noisy measurements.

• Limit external noise by using good quality signal wires.

• Foil shielded with drain wire or dense braided shield; one side only!

• Transition between different thermal environments may require PTFE jacket

• Protect DAS hardware by using reliable fuses (especially on DC side of system)

• Use equipment rated for the worst-case temperature (usually 90°C). 

• Monitoring equipment rated for range of ambient temperatures where deployed. 

• A hardware suite of preference comes with experience (and from failures)

• A trustworthy communications interface and large internal storage is essential. 

“Simple is better” - In our experience this is a true statement.

• Unnecessary signal conditioning increases uncertainty and creates more failure points. 

• Passive devices with low thermal coefficients work very well.

Calibrate

• Failure to do so impacts system rating uncertainty and degradation rates 

• Don’t blindly trust data sheets. 

• Often, the manufacturer calibration is valid for only one year. 

• A complete end-to-end calibration, when possible, limits systematic errors.

• Periodic pyranometer calibrations (such as NREL’s BORCAL) are essential!

• 24 month schedule is normal, periodic inspection / cleaning of sensors
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Keep it Clean

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• Use devices designed and rated for the end-use environment

• Limit access to weather and vermin 

• Use good wire management practices; label signal lines

• Make sure all measurement equipment is grounded 

vs.

Poor wire management; lack of labeling. Corroded signal connections

Lizard living within

enclosure

Degraded polyimide tape; potential 

for poor thermal contact

Clean installation; good wire management; rated devices.

Tight contact with module; limited ability for corners to lift.
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Recommendations

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Topic Recommendation Reason

Measurement Interval 5-sec scans into 1-min averages

15-minute data (standard) cannot capture many system issues; 1-minute data provides 

the resolution needed for capturing transient system events such as inverter shutdown. 

1-minute data also supports analysis which require more data points (degradation rate 

calculations, for instance). With the cost of hardware and storage decreasing there is no 

reason to avoid 1-minute (or sub-minute) data collection.

Analog Signal Lines High quality PTFE Jacket; 

Shielded w/ drain wire

Experience has shown that PTFE jacketed signal lines are less susceptible to noise when 

going between different temperature environments. Also, PTFE has superior resistance 

against weather (UV, moisture, temperature) when used outdoors.

Outdoor Connectors Rated for the environment; 

Use of protective boots.

Use all means to limit connector corrosion. Sunlight resistant boots must be used; 

unrated boots can crack and actually trap moisture within the connector housing.

Equipment

Temperature Ratings -55°C to 85°C Equipment enclosures can see dramatic temperatures, especially if exposed to direct 

sunlight. Equipment must handle these extremes.

Enclosure Locations In the shade Direct exposure to sunlight can raise the interior temperature of an enclosure 

dramatically.

Thermocouple Attachment

Clean location and hands;

Silicone adhesive; 

Round shape

Silicone adhesive has withstood outdoor testing very well. Limit contamination of the 

adhesive by cleaning the attachment location (w/ IPA) and your hands. Avoid touching 

the adhesive by using tape with a split carrier film. Square corners are prone to lifting; 

round dots limits lifting.

Inspection Interval Visual checks quarterly

Check for: 

1)Thermocouple detachment or tape embrittlement

2)Damage to exterior sensors (pyranometers, ambient temp sensors)

3)Evidence of moisture or vermin in enclosures

4)Loose connections at DAS / sensors

Calibration Interval

End-to-End Calibration Annually

-Ambient temp above 15°C

-Wind speeds below 3 m/s

Bench calibrations are done in an atypical environment (indoor is not equivalent to 

outdoor conditions). Calibration equipment can be affected by the surrounding 

environment. Limit temperature extremes and high winds which may cause offsets in the 

calibration hardware. End-to-end calibrations are necessary to limit uncertainties and 

systematic errors. 
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Monitoring Budget

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As budgets for monitoring increase, the measurement data 

sources can be expanded. 

• Allows for additional analysis options and backups
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Summary

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• The dominant error source in determining the system power 

is determining the plane-of-array irradiance

• Performance monitoring is significantly affected by the 

quality of the measurement hardware and the 

professionalism of the installation. 

• Results of performance modeling have greater meaning if 

the measurement hardware is calibrated regularly.

• Calibration drift can invalidate all data since the previous 

calibration.

• No standards for system Energy rating, minimal standards 

for system power rating