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.

Contract No. DE-AC36-08GO28308

Using Accelerated Testing To 

Predict Module Reliability

Preprint

John H. Wohlgemuth and Sarah Kurtz

Presented at the 37th IEEE Photovoltaic Specialists Conference 

(PVSC 37)

Seattle, Washington

June 19-24, 2011

Conference Paper

NREL/CP-5200-50646

July 2011
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1

USING ACCELERATED TESTING TO PREDICT MODULE RELIABILITY

John H. Wohlgemuth and Sarah Kurtz

National Renewable Energy Laboratory, Golden, CO

ABSTRACT

Long-term reliability is critical to the cost effectiveness and 

commercial success of photovoltaic (PV) products. Today 

most PV modules are warranted for 25 years, but there is 

no accepted test protocol to validate a 25-year lifetime. 

The qualification tests do an excellent job of identifying 

design, materials, and process flaws that are likely to lead 

to premature failure (infant mortality), but they are not 

designed to test for wear-out mechanisms that limit 

lifetime. This paper presents a method for evaluating the 

ability of a new PV module technology to survive long-term 

exposure to specific stresses. The authors propose the 

use of baseline technologies with proven long-term field 

performance as controls in the accelerated stress tests. 

The performance of new-technology modules can then be 

evaluated versus that of proven-technology modules. If the 

new-technology demonstrates equivalent or superior 

performance to the proven one, there is a high likelihood 

that they will survive versus the tested stress in the real 

world.

INTRODUCTION

The commercial success of photovoltaics is based on 

long-term reliability of the PV modules that comprise the 

PV systems. Today most PV modules are warranted for a 

25-year life. These modules are typically qualified to either 

IEC 61215 [1] or IEC 61646 [2], which do an excellent job 

of identifying design, materials, and process flaws that are 

likely to lead to premature failure (infant mortality). 

However, the qualification tests are not designed to 

address wear-out mechanisms that will limit module 

lifetime.

Analysis of field data available in the literature indicates 

that the durability of PV modules available commercially 

over the last 10 or even 15 years has been adequate to 

meet the needs of the commercial PV industry. Figure 1 

shows a plot of degradation rate per year versus the 

frequency of that level being reported in the literature [3]. 

This curve represents data from 1920 literature sources. 

The average annual degradation rate is 0.8% per year, 

whereas the most frequently reported value is 0.5% per 

year. So there are many examples of module technologies 

that have proven durability.

However, lower module prices are necessary for the PV 

industry to continue to grow and ultimately reach grid 

parity. The U.S. Department of Energy (DOE) recently 

announced the new SunShot initiative [4], which aims to 

reduce the cost of PV systems to $1/ peak watt, with $0.50 

of this expected to come from the module. This represents 

a significant decrease from the present selling price for all 

PV technologies. Achieving such a large reduction in 

selling price will likely require major changes to the 

module design, materials, and processing. These lower 

module costs must be achieved without adversely 

affecting PV module reliability and durability.

Figure 1. Module degradation rate from the literature

So how can we ensure that changes in design, materials,

and/or processes do not adversely affect module 

reliability? We cannot wait 20 or 25 years to see what 

happens to the lower-cost modules after they are 

deployed in the field. Accelerated reliability tests must be 

used to evaluate the potential of these lower-cost modules 

to survive without increased degradation of output power. 

However no existing accelerated stress tests that we know 

of can determine whether a module type will survive 25 

years wherever it is deployed.

This paper proposes a process that utilizes the past field 

experience of a module type along with comparative 

accelerated stress tests to compare the performance of 

known baseline module types with new-technology, lowercost module types. The new module types are evaluated 

against the older design using accelerated stress tests 

that go beyond those used in qualification test sequences. 

Comparison of the performance between the old and the 

new module types can give an excellent indication of how

well the new design is likely to perform outdoors compared 

to the older design.

LIMITATION OF QUALIFICATION TESTING

Qualification tests are a set of well-defined accelerated 

stress tests developed out of a reliability testing program. 

Qualification tests incorporate strict pass/fail criteria. Such 

tests are used by customers to qualify modules for 

purchase and by manufacturers as a means of 
2

demonstrating a degree of product reliability. These test 

sequences were developed based on the identification of 

field failure mechanisms.

Once typical field failure mechanisms were identified, 

accelerated stress tests were developed that would 

duplicate these failure modes in a reasonably short 

amount of time. Table 1 provides a list of some of the 

accelerated stress tests that have been used along with 

the failure modes that they accelerate. The list in Table 1 

was used to establish the initial PV module qualification 

test sequences that grew into IEC 61215. [5]

Table 1. Accelerated Stress Tests for PV

Accelerated Stress Failure Mode

Thermal Cycle Broken interconnect

Broken cell

Solder bond failures

Junction box adhesion

Module connection open circuits

Open circuits leading to arcing

Damp Heat Exposure Corrosion

Delamination of encapsulant

Encapsulant loss of adhesion & 

elasticity

Junction box adhesion 

Electrochemical corrosion of TCO

Inadequate edge deletion

Humidity Freeze Delamination of encapsulant

Junction box adhesion 

Inadequate edge deletion

UV Test Delamination of encapsulant

Encapsulant loss of adhesion & 

elasticity

Encapsulant discoloration

Ground fault due to backsheet 

degradation

Mechanical Load Broken interconnect

Broken cell

Solder bond failures

Broken glass

Structural failures

Dry and Wet Insulation 

Resistance

Delamination of encapsulant

Ground faults

Electrochemical corrosion of TCO

Inadequate edge deletion

Hot Spot Test Hot spots

Shunts at the scribe lines

Hail Test Broken cells

Broken glass

Bypass Diode Thermal Test Bypass diode failures

Although qualification tests like IEC 61215 and IEC 61646 

are important and valuable, they have limitations because 

the stress levels are by design limited. A significant 

number of commercially available products must be

capable of passing the qualification test sequence or the 

commercial PV business would be severely limited. 

Passing the qualification test means the product has met a 

specific set of requirements, but doesn’t say anything 

about which product is better for long-term performance. 

Most of today’s commercial modules pass the qualification 

sequence with minimal change, meaning that they suffer 

almost no degradation in power output from the test 

sequence. When 10 crystalline-silicon module types, all 

previously qualified to IEC 61215, were tested beyond the 

qualification level (500 versus 200 thermal cycles and 

1250 hours versus 1000 hours of damp heat) only two of 

them passed the extended test [6]. From the results of the 

qualification test there was no way to know that only two

module types out of 10 could pass 1250 hours of damp 

heat without exceeding 5% power loss.

So the qualification test itself could not distinguish 

between the performances of these 10 module types. 

Similarly, the qualification test is not a good tool for 

determining whether a change in materials, processing, or 

design is likely to reduce the module’s lifetime or increase 

the annual degradation rate. However, the qualification 

test sequence can be used to determine that a new design 

will not suffer from infant mortality, and it can provide a 

good starting point for developing a methodology to 

evaluate the impact of new lower-cost designs, materials,

and processes on the modules’ long-term reliability and 

lifetime.

RELIABILITY TESTING

The accelerated testing required for evaluation of the 

impact of cost reducing changes in module construction 

on module durability must address the observed field

failure modes and must cause degradation of the product

to prove that the tests are addressing the same failures as 

seen in the field. Because the accelerated stress tests 

from the qualification tests are designed to address the 

identified field failure modes, they are a good starting point 

for developing these reliability tests. How can the 

qualification tests be turned into reliability tests? The 

following approaches may all contribute to the

development of a test sequence evaluating a module’s 

ability to survive for 25 years in a particular terrestrial 

environment.

• Increase the test duration, for example do more 

thermal cycles or expose the modules to damp heat for 

a longer time.

• Use higher stress levels, but making sure that the 

higher stress levels do not cause failures that are not 

seen in the field. 

• Combine stresses, for example by applying voltage to 

the module during damp heat.

• Evaluate new methods to accelerate the failure modes 

identified in the field, for example by using the dynamic 

mechanical load test to accelerate cell breakage 

caused by wind-induced vibrations [7].

• Use material or coupon tests in situations where it 

would be too expensive to test full-sized modules. For 

example, long-term ultraviolet (UV) testing at high 

temperature to evaluate material discoloration and 

degradation is better performed on small coupons of 

the same cross-sectional construction as the module.

• Use step stresses in which the initial stress starts with 

the stress level from the qualification test and 

increases until failures are seen. Once again, care 
3

must be taken to ensure that the failures seen are the 

same failure modes identified from field exposure.

In the qualification tests, pass/fail measurements include 

peak power at Standard Test Conditions (STC) and the

dry and wet leakage currents. The first way to increase the 

available information about new lower-cost technologies is 

to eliminate pass/fail and report the actual performance so 

that products can be compared directly with each other. 

There are additional measurement tools that can be used 

to observe problems, often before they impact the power 

or leakage currents. The following measurement tools can 

be valuable for identifying failure modes before they are 

serious enough to cause measureable power loss.

• Visual inspection can be used to observe 

discoloration of encapsulant, corrosion of metals, and 

delamination of the encapsulant or backsheet.

• Infrared (IR) cameras show heat dissipation, so they

can be used to determine areas where collection of 

current has been disrupted (higher series resistance) 

or where current is flowing where it shouldn’t 

(shunting). An IR camera can also be used to show 

whether the current path through a string of cells is 

intact and whether the module bypass diodes are 

carrying current during normal operation or only

during shaded operation as designed.

• Electroluminescence looks at the near-IR light 

generated by carriers transitioning across the cell p-n 

junction. So electroluminescence can be used to see 

discontinuities in the junction such as cracks in the 

cell or breaks in the junction itself.

• When testing for adhesion of the package layers, the 

junction box and the frame can identify possible 

weaknesses in the packaging technology.

• Dark I-V curves can identify small changes in series 

and shunt resistance before they are large enough to 

change the light I-V curve appreciably.

A number of publications have suggested adopting more 

aggressive test protocols. Some of these have been 

proposed by PV module manufacturers [8 and 9], some by 

test laboratories [10], and others by groups organized by 

customers [11]. In most cases, they call for increasing the 

test durations. Although most of these suggested test 

protocols are based on equating field experience with 

results of accelerated testing, none purport to predict 25-

year life.

An effort is now underway to establish a methodology for

making the accelerated stress tests more quantitative [12].

The long-term goal is to determine the acceleration rates 

to use for many of the long-term wear-out mechanisms. 

For some, such as thermal cycling of crystalline-silicon 

modules, existing analyses of field data [13] and modeling 

[14] of weather data can be used to provide an estimate of 

the acceleration rate and therefore give a better estimate 

of the number of thermal cycles we should run for a 25-

year lifetime.

For many of the other long-term stresses that lead to 

module wear-out, the appropriate acceleration factors are 

unknown. The impacts of combined stresses on the 

module lifetime are also unknown. Therefore, both realtime and accelerated stress tests are needed in order to 

evaluate the changes made to modules and to develop 

tests that can help us elucidate the modules’ long-term 

performance in the variety of terrestrial environments in 

which they are deployed.

ESTABLISHING A BASELINE

We do not know how to test modules for a 25-year 

lifetime, but we cannot wait 25 years to find out how they

will perform. We do have module types that have already 

survived long time periods, in some cases more than 25 

years in the field with acceptable degradation rates. These 

module types can provide a valuable baseline. Analyses 

of modules that have survived for a long time in the field

have provided a great deal of data on failure rates, 

degradation rates, and failure modes [15]. The materials 

group of Working Group 2 (Modules) of IEC TC-82 on PV 

has begun an effort to evaluate modules that have been 

exposed in the field for long time periods. In parallel with 

this effort, new modules and samples are being 

constructed using materials, processes and designs that 

are as similar as possible to those used in construction of 

the original fielded modules. These new modules are then 

subjected to various accelerated stress tests to see what 

levels of accelerated stress duplicate the field failures.

However, there is one more way to gain knowledge from 

the baseline module types. Because they can survive for 

long time periods, their construction can be used as a 

baseline in accelerated stress testing. For example, 

assume that a module manufacturer wants to change 

encapsulants from one with proven field performance that 

the company has been using for years to a new-lower cost 

material. What testing should be performed? The first step

would be to perform the qualification tests on the new 

material. Assuming it passes that without problems, the 

second step is to test both the new material and the old 

material through more severe environmental tests (with 

guidance given in the next section) in an attempt to 

duplicate wear-out mechanisms. The results from the old 

and new encapsulant can then be compared. If the new 

encapsulant performs as well as the old one in the

appropriate accelerated stress tests, then it is likely to 

perform as well as the old material in the field.

DEVELOPING THE EXTENDED TEST SEQUENCE

Thermal Cycling

The 200 thermal cycles in the Qualification test does not

appear to represent 25 years outdoors in most terrestrial 

environments [13 and 14] depending on the site. Various 

levels of cycles (from 400 up to 1000) have been 

proposed by different authors. Interestingly, it has been 

shown that well designed and constructed modules can 

survive 1500 thermal cycles without appreciable power 

loss. [16] To assess a new technology’s ability to survive 

thermal cycles, it should be cycled along with the control 
4

module type until either one or the other begins to degrade 

or to some preset number of cycles many times the 

qualification level. Assessment of performance should be 

based on a comparison of both the change in output 

power and on observations of detrimental changes (e.g.

broken interconnects). If the two technologies exhibit 

similar thermal cycle performance, then the two 

constructions should have similar field performance for 

failure modes caused by thermal cycling.

Damp Heat

The 85°C/85% relative humidity exposure is as 

accelerated as necessary and probably at the practical 

limit for acceleration of damp heat. These conditions 

probably never happen in the real world as the modules 

tend to dry out at their highest temperatures, but absorb 

moisture at lower temperatures. It is difficult to judge what 

outdoor exposure the 1000-hour exposure at 85/85 

represents particularly as different module designs will 

experience very different stress from this exposure. So 

rather than specify a particular length of time, it seems 

appropriate to test the control technology and the new 

technology through enough hours of damp heat that both 

begin to lose some power (say to 90% of the original) in 

order to verify that the new technology is no worse and 

has no additional failure modes than the old module 

technology it will replace.

UV/TC50/HF10 

This sequence of tests (UV exposure, followed by 50 

thermal cycles and 10 humidity-freeze cycles) is mainly a 

test for the package. The module failing this test indicates 

inadequate adhesion between layers or inadequate cure 

level in the encapsulant. Humidity freeze is not really a 

lifetime test for a particular wear out mechanism so 

typically does not need to be enhanced for reliability 

testing. Although some people have recommended 

repeating the humidity freeze test a number of times, it is 

not clear what wear-out failure modes they are trying to 

accelerate. So our recommendation is to test both the new 

and the old module construction to the sequence defined 

in IEC 61215/61646, but with the addition of the dynamic 

mechanical load test, which is discussed in the following 

section.

Dynamic Mechanical Load Test 

The only mechanical test in IEC 61215 is a static 

mechanical load test that is performed after the 

accelerated stress tests. A dynamic mechanical load test 

followed by 50TC/10HF does a much better job of 

identifying modules with cells that are prone to breakage 

and would cause subsequent power loss [7]. There is an 

available DIN Standard (EN12211) [17] that can be used 

for this test until IEC Working Group 2 can finish 

preparation of a dynamic mechanical load test for PV 

modules.

UV Material Test 

Although IEC 61215 contains a UV test, it is intended only 

as a pre-screening test to address UV-sensitive bonding 

issues. This test is not long enough to assess whether the 

polymeric materials used in a module are capable of 

surviving the UV exposure expected during the lifetime of 

the module. Specialized Technology Resources, Inc. 

(STR) developed a long-term UV exposure protocol during 

its work evaluating the causes of EVA yellowing [18]. BP 

Solar reported the use of a similar UV exposure protocol 

for 26 weeks to verify a 25-year lifetime [19]. A similar UV 

testing protocol should be used to evaluate any new 

polymeric material for use in a PV module. The material 

should be exposed to the UV within the standard package

in which it will be used. Because there is no agreement 

between UV dose and years in the field, it would be best 

to perform the test with the new material side-by-side with 

the material it is to replace. The test should proceed for at 

least the proposed 26 weeks or until one or both of the 

materials begin to discolor or degrade. At that point, a 

comparison between the old and new materials will 

indicate whether the new material will perform as well as 

the one it is design to replace.

Potential Induced Degradation

There have been recent reports of significant module 

degradation associated with being mounted at the 

negative-voltage end of a high-voltage string [20]. The 

failure mechanism appears to be related to voltageinduced ionic current flow, which is made worse in the 

presence of high humidity or liquid water. Preliminary 

results indicate that some module types are susceptible to 

this degradation mechanism, whereas others are not. 

Although no standard test has been developed for this 

failure mode, adding a combined 

temperature/humidity/voltage-bias component to the test 

sequence should verify that the new-technology module 

type doesn’t introduce this failure mode.

CONCLUSIONS

A method for assessing the long-term reliability and 

durability of new lower-cost PV modules has been 

proposed. Because the new approach compares the 

results of the accelerated testing with modules that have a 

known long lifetime, a new module type qualified through 

this procedure has a high likelihood of also being able to 

survive in the field. The recommendations given in this 

paper can serve as a guideline for the establishment of a 

specific program of reliability testing for each major costreduction proposed. Use of this methodology will reduce 

the risk that a change made to lower costs will have a 

major, negative impact on module lifetime or degradation 

rate.

Going forward, this method can also provide valuable 

input into the development of accelerated test procedures 

that can predict 25-year lifetime.
5

ACKNOWLEDGMENT

This work was supported by the U.S. Department of 

Energy under Contract No. DE-AC36-08-GO28308 with 

the National Renewable Energy Laboratory.

The authors wish to thank Dirk Jordan for providing the 

data on module degradation rates in Figure 1.

REFERENCES

[1] IEC 61215 “Crystalline silicon terrestrial photovoltaic 

(PV) modules – Design qualification and type approval”.

[2] IEC 61646 “Thin-Film terrestrial photovoltaic (PV) 

modules – Design qualification and type approval”

[3] D. C. Jordan and S. R. Kurtz, “Thin-Film Reliability 

Trends toward Improved Stability”, 37th IEEE PVSC,

2011.

[4] DOE SunShot Initiative. Retrieved May 23, 2011 from 

http://www1.eere.energy.gov/solar/SunShot

[5] John Wohlgemuth, “Overview of Failure Mechanisms 

and PV Qualification Tests”, Proceedings of the PV 

Module Reliability Workshop, Golden, CO, 2010, 

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http://www1.eere.energy.gov/solar/pv_module_reliability_

workshop_2010.html

[6] J. H. Wohlgemuth, D. W. Cunningham, D. Amin, J. 

Shaner, Z. Xia, and J. Miller, “Using Accelerated Tests 

and Field Data to Predict Module Reliability and Lifetime”, 

23rd EU PVSEC, 2008, p. 2663.

[7] John H. Wohlgemuth, Daniel W. Cunningham, Neil V. 

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of Cell Thickness on Module Reliability”, 33rd IEEE PVSC, 

2008.

[8] John Wohlgemuth “Long Term Photovoltaic Module 

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[9] Bethanne Felder and Sonya Xu, “Qualification and 

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[10] Allen Zielnik, “PV Durability and Reliability Issues”,

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[11] Alelie Funcell, “The ‘Thresher Test’”, Proceedings of 

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[12] International PV Module QA Workshop

Environment-Specific Module Durability Testing

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http://www.nrel.gov/ce/ipvmqa_forum/index.cfm.

[13] J.H. Wohlgemuth, D. W. Cunningham, A.M. Nguyen 

and J. Miller, “Long Term Reliability of PV Modules”, 20th

EU PVSEC, 2005.

[14] Nick Bosco and Sarah Kurtz, “Quantifying the 

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workshop_2010.html.

[15] D. C. Jordan and S. R. Kurtz, “PV Degradation Rates 

– An Analytical Review”, submitted to Progress in 

Photovoltaics: Research and Applications, 2011.

[16] John Wohlgemuth “Module Technology: Current 

Practice and Issues”, MRS Workshop on Photovoltaic 

Materials and Manufacturing Issues, Denver, CO. Oct, 

2010.

[17] DIN EN12211 “Windows and doors resistance to wind 

load – test method”

[18] William H. Holley, Jr, Susan Agro, James P. Galica, 

Lynne A. Thoma, Robert S. Yorgensen, “Investigation into 

the causes of browning in EVA encapsulated flat plate PV

modules”, 24th IEEE PVSC, 1994, pp. 893-896.

[19] John H. Wohlgemuth, Daniel W. Cunningham, Paul 

Monus, Jay Miller, and Andy Nguyen “Long Term 

Reliability of Photovoltaic Modules”, 4th WPEC, 2006.

[20] Peter Hacke, “Degradation Mechanisms of PV 

Modules at Systems Voltage”, Proceedings of Fraunhofer 

ISE PV Module Reliability Workshop, Berlin, 2011.