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.

Questions Utilities Should Ask to Mitigate PV 

Technology Risk 

Utility/Lab Workshop on 

PV Technology and 

Systems 

John Wohlgemuth

November 9, 2010

Tempe, Arizona

NREL/PR-5200-49929
NATIONAL RENEWABLE ENERGY LABORATORY

Introduction – Questions

• What do utilities need to know about a PV system, 

especially the modules before they invest in the 

system?

• What model is being utilized to predict the 

performance?

•What weather data has been utilized to predict the 

performance?

•How will the performance vary with time?

• Permanent changes

• Long term degradation

• Failure rate

• Transient changes

•What O&M is necessary to obtain the modeled 

performance?

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Module should be

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• Be qualified to IEC 61215 or 61646.

• Be safety certified to UL 1703.

• Be safety certified to IEC 61730 parts 1 and 2.

• Have at least a Class C fire rating if it is going to be 

used on or near a building.

• Warranted for at least 25 years

• Characterized under all expected operating 

conditions.
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Module Performance – Typically Supplied

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STC Values of

– Pmax Isc Voc Ipmax Vpmax

Temperature Coefficients of Isc, Voc and Pmax

Maximum Systems Voltage (600 for US, 1000 for ROW)

NOCT – Cell temp at 800 W/m2, 20 C ambient, 1 m/s ws

Fuse Rating and maximum reverse current

Tolerance of power (but not of other parameters)

May get performance at NOCT

May get performance at low irradiance (200 W/ m2)

These values are required by either IEC 61215 or 61646, IEC 

61730 and/or EN 50380 (European standard for Module 

Nameplate and Datasheet)

(Show example Module Spec Sheet)
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• The first question should be “Does the STC power meet 

the specification”?

• A number of publications have pointed out that the most 

important factor effecting the energy collection (kWhr/kWp) 

is the Actual Module Power versus the Nameplate Module 

Power. 

(See for example Ransome & Wohlgemuth, “An Overview 

of 4 years of kWhr/kWp Monitoring at 67 Sites Worldwide”, 

WCPEC-3 , 2003.)

• So ask for 3rd party verification of module power 

measurements or even make it a part of the procurement,

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Do the Modules Meet the Specification?
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Why do you need additional Module Information? 

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Pmax is a non-linear function of irradiance. 

Typical PV sizing programs use the following steps:

– Calculate tilted plane hourly irradiance from monthly 

horizontal plane insolation data.

– Estimate module temperature from the NOCT value.

– Generate hourly Pmax as a function of irradiance and 

ambient temperature using a 1-diode model.

– Estimate other modules losses (soiling, shading, mismatch, 

snow, etc.)

– Estimate systems losses (inverter efficiency, Vmax tracking 

and wiring losses)

– Sum results to get expected energy collection.
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Issues

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Monthly average data changes the distribution of the 

irradiance versus insolation curve.

– Modules actually operate at higher irradiances than monthly 

average predicts. ( See figure)

NOCT temperature models are purely linear with 

respect to irradiance.

– Better thermal models are available & you can do things to 

make PV modules run cooler. (See figure)

1-Diode model does not do a good job of predicting PV 

module performance as a function of irradiance. 

– Where do you get the parameters to use in the model? (See 

Figure)
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Monthly Average vs Hourly vs 5 minute 

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Ransome & Funtan: “Why hourly averaged measurement data is 

insufficient to model PV system performance accurately”, 20 PVSEC, 2005 
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Changing the thermal performance

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Thermocool Technology – Z. Xia, BP Solar
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Modeled versus Measured Performance

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Wohlgemuth, etal, 34th IEEE PVSC 2009
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What should you ask for?

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Measured performance as a function of irradiance and 

temperature. Can use

– Matrix approach from IEC 61853-1 “Irradiance and Temperature 

Performance Measurements and Power Rating”

– Empirical model based on measured outdoor performance such 

as King model or Ransome model which give good results over 

long time periods but may not at specific times. 

(See Kroposki, Marion, King, Boyson & Kratochvil “Comparison 

of module performance characterization methods” 28th IEEE 

PVSC, 2000.)
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Matrix from IEC 61853-1

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Isc, Pmax, Voc and Vmax versus Irradiance and Temperature 

IRRADIANCE Spectrum Module Temperature

W-m-2 15 °C 25 °C 50 °C 75 °C

1100 AM1.5 NA

1000 AM1.5

800 AM1.5

600 AM1.5

400 AM1.5 NA

200 AM1.5 NA

100 AM1.5 NA NA
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What should you ask for?

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Thermal model for performance as a function of irradiance, 

ambient temperature, wind speed and mounting geometry.

See for example:

E. Skoplaki, J.A. Palyvos, Operating temperature of photovoltaic 

modules: A survey of pertinent correlations, Renewable Energy 34 

(2009) 23–29.

D.L. King, W.E. Boyson, and J.A. Kratochvil, Photovoltaic Array 

Performance Model, Sandia National Laboratories, SAND2004-3535, 

(2004).

D. Faiman, Assessing the Outdoor Operating Temperature of 

Photovoltaic Modules, Prog. Photovolt: Res. Appl. 16 (2008) 307–315.

IEC 61853-2 “Spectral Response, Incidence Angle and Module 

Operating Temperature Measurements” 
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Warranties

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Typical Module Warranty

– 90% power warranty for 12 years

– 80% power warranty for 25 years.

New Warranty for Large Systems Purchase

– Initial tolerance and LID accommodation (~3%)

– Then annual degradation rate (~ 0.5% to 0.7%/year)

See Figure
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Linear versus Stepped Warranty

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From SolarWorld web site
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What is behind the warranty?

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Ask to see field data on returns and power degradation.

– What percentage of modules are returned on an annual basis?

– How many modules and systems is the degradation rate based 

on?

– How many years have the modules and systems been 

deployed?

– Where have modules and systems been deployed?

Ask to see accelerated stress/reliability test data to support 

the lifetime and degradation rate predictions.

– The Qualification Test levels from IEC 61215 and IEC 61646 are 

not enough. These check design and infant mortality.

– Need more thermal cycles, longer damp heat and UV testing of 

materials.
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Testing Cry-Si Modules beyond IEC

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Manufacturer DMLC/TC/HF 1250 Hr DH 500 TC

A Pass Fail Fail

B Pass Fail Pass

C Pass Fail Fail

D Pass Fail Pass

E Pass Fail Pass

F Pass Pass Pass

G Pass Fail Pass

H Pass Pass Pass

J Pass Fail Pass

K Pass Pass Pass

Wohlgemuth, etal 23rd PVSEC, 2008
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Performance Models – Ask for Validation

Compare initial model results obtained 

using modeled irradiance with both 

modeled results using measured 

irradiance and measured performance 

using measured irradiance.

The example from Ransome (33rd IEEE 

PVSC, 2008) shows the differences 

between calculated and measured 

energy yields using measured and 

modeled values of irradiance but 

correcting the total modeled insolation

to be the same as that measured.

In this case about half of the 3% 

difference is due to how the model 

deals with the irradiance and about half 

due to how the model deals with the 

module. 

Modeled

Performance

Measured

Performance

Modeled

Irradiance

97% 98.9%

Measured

Irradiance

98.4% 100%

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Weather – What may lead to problems in predicting performance

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High levels of diffuse light.

Localized conditions – ground fog & mist, clouds coming off 

the water

Rapidly moving cloud cover

Long periods without rain can lead to soiling losses

In windy areas modules will run cooler than predicted and 

so produce more energy
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SUMMARY

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Accurately predicting the energy production of a PV system over 

its 25 year life requires: 

– The use of a good validated PV model. 

– Full characterization of the performance of the modules to be used.

– Accurate prediction of the expected module degradation rate.

– Definition and implementation of an O&M procedure to identify and 

replace the small fraction of failed modules expected. 

– Definition and implementation of an O&M procedure to handle 

problems with BOS components.

Careful monitoring of the system output versus weather 

parameters can provide an early indication of problems so they 

can be remedied before they lead to large losses in energy 

production.
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Summary - So Ask For Modules with

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• 3rd party certifications to IEC 61215 or 61646, IEC 61730-

1 and 2 and UL1703.

• 3rd party certification of module STC Pmax

• Measured performance as a function of irradiance and 

temperature (probably from 3rd party)

• Warranty with an annual degradation rate.

• Field and accelerated test data supporting the module 

warranty.