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SolarCity Photovoltaic Modules with 35 Year Useful Life Andreas Meisel 1 , Alex Mayer 1 , Sam Beyene 1 , Jon Hewlett 1 , Karen Natoli Maxwell 1 , Nate Coleman 1 , Frederic Dross 2 , Chris Bordonaro 3 , Jenya Meydbray 2 , Elizabeth Mayo 2 1) SolarCity, 161 Mitchell Blvd, Suite 104, San Rafael, CA 94903 2) DNV GL, 1360 Fifth Street, Berkeley, CA 94710 3) DNV GL – Renewables Advisory, 155 Grand Ave, Oakland, CA 94612

SolarCity Photovoltaic Modules with 35 Year Useful Life...SolarCity defines 35 year Useful Life as 95% of mod-ules producing at least 80% of their power after 35 years in their use

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Page 1: SolarCity Photovoltaic Modules with 35 Year Useful Life...SolarCity defines 35 year Useful Life as 95% of mod-ules producing at least 80% of their power after 35 years in their use

SolarCity Photovoltaic Modules with 35 Year Useful Life

Andreas Meisel1, Alex Mayer1, Sam Beyene1,

Jon Hewlett1, Karen Natoli Maxwell1, Nate Coleman1,

Frederic Dross2, Chris Bordonaro3, Jenya Meydbray2,

Elizabeth Mayo2

1) SolarCity, 161 Mitchell Blvd, Suite 104, San Rafael, CA 94903 2) DNV GL, 1360 Fifth Street, Berkeley, CA 94710 3) DNV GL – Renewables Advisory, 155 Grand Ave, Oakland, CA 94612

Page 2: SolarCity Photovoltaic Modules with 35 Year Useful Life...SolarCity defines 35 year Useful Life as 95% of mod-ules producing at least 80% of their power after 35 years in their use

SolarCity Photovoltaic Modules with 35 Year Useful Life

Andreas Meisel1, Alex Mayer1, Sam Beyene1, Jon Hewlett1, Karen Natoli Maxwell1, Nate Coleman1,

Frederic Dross2, Chris Bordonaro3, Jenya Meydbray2, Elizabeth Mayo2

1) SolarCity, 161 Mitchell Blvd, Suite 104, San Rafael, CA 94903 2) DNV GL, 1360 Fifth Street, Berkeley, CA 94710

3) DNV GL – Renewables Advisory, 155 Grand Ave, Oakland, CA 94612

Table of Contents SolarCity Photovoltaic Modules with 35 Year Useful Life ............................................................................. 1

1 Executive Summary ............................................................................................................................... 3

2 Useful Life – Introduction ..................................................................................................................... 3

3 SolarCity Total Quality Control Program ............................................................................................... 5

3.1 Rigorous Supplier Selection and Three-Level Oversight Process .................................................. 5

3.2 Stringent Module Quality Specifications ...................................................................................... 5

3.3 Effective Prevention of Quality Deviations ................................................................................... 6

3.4 Constant Refinements and Total Integration ............................................................................... 6

3.5 World-Class Team behind the Scenes ........................................................................................... 7

4 Useful Life Extrapolation from Accelerated Testing ............................................................................. 7

4.1 Ongoing Reliability Testing – Overview ........................................................................................ 7

4.2 ORT data – Thermal Cycling .......................................................................................................... 8

4.3 ORT data – Damp Heat, Humidity Freeze, and Dynamic Mechanical Load Testing ................... 10

4.4 PQP Testing – Testing Beyond Standard Qualification Tests ...................................................... 13

4.5 Product Qualification Program Testing – Overview .................................................................... 14

4.6 Product Qualification Program Testing – Extended Thermal Cycling ......................................... 14

4.7 Product Qualification Program Testing – Extended Damp Heat ................................................. 17

4.8 Product Qualification Program Testing – PID Testing ................................................................. 18

4.9 Product Qualification Program Testing – Extended Humidity Freeze and UV Test .................... 19

5 Next Steps – Tests with Improved Correlation to Real Life ................................................................ 19

6 Useful Life Extrapolation Based on Degradation of Fielded Modules ................................................ 20

7 Conclusion ........................................................................................................................................... 21

8 References .......................................................................................................................................... 21

Page 3: SolarCity Photovoltaic Modules with 35 Year Useful Life...SolarCity defines 35 year Useful Life as 95% of mod-ules producing at least 80% of their power after 35 years in their use

3 SolarCity Photovoltaic Modules with 35 Year Useful Life

Executive Summary

SolarCity believes that the Useful Life of the photo-

voltaic (PV) modules that are being installed on its

residential and commercial systems is 35 years or

longer. Per this definition, 95 % of all modules in-

stalled are expected to have an annual average deg-

radation rate of less than ~0.5 % and produce at

least 80 % of their power after 35 years of service.

Experimental data from accelerated stress tests ac-

cording to the industry standard IEC 61215, which

were performed by DNV GL (the leading US certified

3rd

party), demonstrates that the median power deg-

radation of modules supplied by seven key SolarCity

approved module manufacturers for all tests and all

module suppliers combined is as low as -1.1 % and

as much as 35 % lower than for a comparable indus-

try-wide selection of non-SolarCity modules meas-

ured at DNV GL. Furthermore, data from DNV GL

demonstrates that after extending accelerated test-

ing to more than 3x beyond the conditions of IEC

61215, the modules produced for SolarCity show

only 1 to 2 % median degradation and outperform

non-SolarCity modules, which are typically warrant-

ed for 25 years.

The reason for this advantage is SolarCity’s imple-

mentation of a stringent and industry-leading Total

Quality Program, which adopted its features from

the Automotive Industry and was implemented by

SolarCity in early 2014. Following this program, So-

larCity strategically chooses to engage with a select

group of Tier-1 suppliers only. In order to be quali-

fied as a SolarCity supplier, manufacturers need to

have effective Quality Assurance programs and re-

fined manufacturing processes in place, and steady

product and manufacturing quality must be demon-

strated. Rigorous tests need to be passed on an on-

going basis, performed by a qualified 3rd

party lab.

Furthermore, we require that factory controls and

in-line testing are in place to ensure quality is sus-

tained over time and deviations are rapidly detected,

so the deployment of faulty products in the field is

prevented. Additional work is underway to demon-

strate that the degradation rate from SolarCity mod-

ules in the field is lower than industry-standard. Last-

ly, the development and implementation of state-of-

the-art accelerated testing methods will enable So-

larCity to probe degradation modes that are not de-

tectable with the current industry-standard suite of

testing and to more reliably predict real-life perfor-

mance in the field.

The most comprehensive meta-study of Field Degra-

dation rates to date, where more than 11,000 annual

degradation rates have been aggregated and ana-

lyzed, observed a near-linear degradation behavior

for the majority of crystalline-Silicon (Si) modules

and established a median degradation rate for Si

modules of around 0.5 % per year [1, 2]. The data in

this study was analyzed and filtered by DNV GL ana-

lysts, and the annual median degradation rate for

crystalline-Si modules was confirmed to be ~0.50 %

per year, while the corresponding value determined

for systems is 0.77 % per year [3].

The data presented in the following supports the

assumption that SolarCity’s PV modules, as a result

of its Total Quality Program and advancements in

Materials Science, manufacturing, and quality con-

trol, perform at least similar, if not better than the

median of all crystalline-Si modules observed in the

study above. Therefore, an annual module degrada-

tion rate of 0.5-0.6% per year is a realistic assump-

tion, which warrants a postulation of Useful Life of

35 years with a power output of 80 to 82.5 % there-

after.

1 Useful Life – Introduction

SolarCity defines 35 year Useful Life as 95% of mod-

ules producing at least 80% of their power after 35

years in their use environment [4]. ‘Use environ-

ment’ is defined as all geographic and meteorologi-

cal conditions that the PV modules will experience

during their lifetime. Site environmental conditions,

installation, and handling are included in use-

environment considerations. This definition of Useful

Life postulates a higher threshold for the remaining

power output than the value of 70 % that has been

assumed elsewhere [5].

Industry analysists have been getting more comfort-

able with the idea of Useful Life beyond 30 years [6].

The Useful Life of a PV module is determined by

Page 4: SolarCity Photovoltaic Modules with 35 Year Useful Life...SolarCity defines 35 year Useful Life as 95% of mod-ules producing at least 80% of their power after 35 years in their use

4 SolarCity Photovoltaic Modules with 35 Year Useful Life

wear-out failures, which occur at the end of the

working lifetime of the module. SolarCity defines the

end of a PV module’s Useful Life if a safety problem

occurs or if the module power drops below 80 % of

the initial power rating. Long-term studies that have

investigated wear-out failures [7] found that the

predominant End-of-Life failures led to a median

power loss of only 10 % (between 0 % and 20 %),

and that nearly all of these PV modules were still

functional and met the manufacturer’s power war-

ranty. Another literature meta-study summarizing

~400 reports on degradation rates of silicon modules

confirms that modules are usually observed to de-

grade slowly in the field [8]. The degradation most

often is dominated by a gradual loss of short-circuit

current, which is mostly associated with discolora-

tion and/or delamination of the encapsulant materi-

al. In other words, the most critical module failures

have been observed to occur relatively fast, whereas

modules that do not show early failures are likely to

reach the wear-out portion of the ‘bathtub’ product

reliability curve, where the power declines in a grad-

ual and slow manner rather than showing abrupt

failure [9, 10].

There are numerous examples of installations that

have delivered stable performance for well over

25 years [11]. In 1984, Sweden’s first grid-connected

photovoltaic system was built in Stockholm. Since its

installation, the 2.1 kW system has been continuous-

ly and reliably producing energy – with less than 3 %

change since the system was installed 31 years ago.

Another system installed in 1984 is at Kyocera’s Sa-

kura Solar Energy Center near Tokyo. The 43 kW

array continues to generate a stable amount of elec-

tricity today 32 years later [12].

Given the drastic advancements in terms of Materi-

als Science, manufacturing processes, quality control

and standards, and theoretical understanding over

the last 30+ years, it is considered reasonable to as-

sume that the quality and reliability of modules fab-

ricated over the last few years can have Useful Life

well beyond 35 years, provided that adequate quali-

ty assurance measures, such as SolarCity’s Qualifica-

tion Program, have indeed been implemented.

The most comprehensive study of Field Degradation

rates to date, where more than 11,000 annual deg-

radation rates have been aggregated and analyzed,

established a median degradation rate for crystal-

line-Silicon (Si) modules of consistently around 0.5 %

per year (Figure 1) [1-3]. The data presented in the

following supports the assumption that SolarCity’s

PV systems, as a result of its Total Quality Program

and industry-wide advancements in Materials Sci-

ence, manufacturing, and quality control, perform at

least similar, if not better than the median of all

crystalline-Si systems observed in the study above.

Therefore, an annual module degradation rate of

0.5 % per year is a realistic assumption, which war-

rants a postulation of Useful Life of 35 years with a

power output of 82.5 % thereafter.

Figure 1 [Taken from Ref 1]: Histograms of all data, high

quality data and the median per study and system pre-

sented as the normalized frequency (a). Cumulative distri-

bution functions for high-quality x-Si systems and modules

(b). The median is indicated by a dashed horizontal line,

0.5 %/year and 1%/year degradation are indicated as a

dashed and dash-dotted line, respectively. The number of

data points for the respective subsets is given in parenthe-

ses.

Modules, all (1552)Systems, all (385)Modules, median (61)Systems, median (71)

Cu

mu

lati

ve p

rob

abili

ty

Degradation rate (%/year)

No

rmal

ized

Fre

qu

ency

(a)

(b)

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5 SolarCity Photovoltaic Modules with 35 Year Useful Life

2 SolarCity Total Quality Control Program

2.1 Rigorous Supplier Selection and Three-Level Oversight Process

SolarCity has taken on an industry thought-

leadership position in the field of Quality and Relia-

bility (Q&R) by developing and implementing a Total

Quality Control Program, which is unique in its depth

for the solar industry (Table 1). Its features were

adopted from the Automotive Industry, and it was

implemented by SolarCity in early 2014.

Table 1: Comparison of Quality and Reliability practices for

typical solar installers and SolarCity, respectively.

The Total Quality Program starts with a stringent

selection process to establish its product suppliers.

SolarCity chooses strategically to only engage with a

select group of Tier-1 suppliers that have effective

Quality Assurance programs and refined manufac-

turing processes with well-controlled Bills of Materi-

als (BOM), which are thoroughly tested to rigorous

standards. Tier 1 manufacturers are required to in-

vest heavily in R&D, use highly automated manufac-

turing techniques and have at least five years history

of producing solar panels. By exclusively selecting

strategic Tier1 suppliers, SolarCity demonstrates its

unconditional commitment to not compromise qual-

ity and reliability in the pursuit of ever lower cost

targets, which is in contrast to numerous competi-

tors who have been plagued by serious quality prob-

lems. For example, significant defect rates of PV

modules were detected during audits of 50 Chinese

factories between 2012 and 2013 [13]. SolarCity has

been working relentlessly with suppliers to ensure

that SolarCity products are free of such defects.

SolarCity’s commitment to quality is reflected in the

fact that Q&R requirements are directly embedded

into and enforceable through the Master Procure-

ment Agreements (MPA) for its product suppliers. In

order to be qualified as a SolarCity supplier, suppli-

ers are contractually required to subscribe to a well-

documented three-level process: (1) Initial vendor

qualification, which requires demonstrating the ca-

pability to manufacture the products according to

well defined specifications and quality requirements,

to pass an onsite factory audit, and to pass reliability

testing through a chosen 3rd

party lab (DNV GL); (2)

Continuous Production Oversight, which ensures

consistent production of goods of high quality by

means of regular BOM inspections and factory audits

through SolarCity as well as 3rd

party auditors; (3)

Ongoing Quality Assurance and Testing to ensure

compliance with the Initial Qualifications and all

quality criteria.

The Total Quality Program was first implemented for

PV modules and has since been extended to all key

components of the entire photovoltaic system. It is

now also in place for inverters, which often are con-

sidered the central part of the PV system, as well as

for pre-installed and field-made PV connectors,

which are another essential system component. Ad-

ditionally, the program has been implemented for

battery & storage and gateways. This three-pronged

Quality Program that is in place for all relevant sys-

tem components ensures that SolarCity products are

designed for long-term reliability and consistent per-

formance over the entire system life exceeding

35 years.

2.2 Stringent Module Quality Specifica-tions

SolarCity has developed a rigorous testing program

of all the components in the PV system, which goes

well beyond the common practices in the solar in-

dustry space. The Module Quality Specification,

which is part of the initial vendor qualification, was

developed with input from over half a dozen Tier-1

module suppliers and standardizes well-defined

module quality requirements across all vendors to

Conventional Method SolarCity Method

Choose suppliers offering lowest cost No supplier qualification Buy off the shelf

☺ Tier-1 suppliers only

☺ Rigorous supplier qualification

☺ Design to spec

Implicit Quality/Reliability

☺ Automotive-based Q&R programs:

For all key system components

☺ Define/control/validate

☺ Gates with sign-off, involvement

No feedback loops No learning from past experiences

☺ Constant feedback between

Installers, O&M, Engineering

☺ Analyze field and warranty data

No Quality philosophy ☺ Company-wide Quality philosophy

☺ Industry thought-leadership in Q&R

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6 SolarCity Photovoltaic Modules with 35 Year Useful Life

ensure consistency and highest quality in SolarCity’s

products.

Product Qualification Program (PQP) Testing and

Ongoing Reliability Testing (ORT) are implemented in

parallel to ensure that performance parameters

achieved on the first set of modules qualified are

reliably sustained over time. PQP and ORT testing

are performed for every single BOM variation per

module supplier. The extended testing conditions of

the Product Qualification significantly exceed indus-

try common test standards. Besides extended test

durations and exposure conditions for well-

established tests, SolarCity also requires an addi-

tional salt-mist corrosion certification for materials,

as well as reliable PID resistance under most aggres-

sive test conditions for up to 600 hours. Further-

more, in order to obtain product certification, mod-

ule suppliers are required to not only meet UL (Un-

derwriter Laboratories) PV module standards, but at

the same time also IEC (International Electrotech-

nical Commission) standards. This leads to additional

quality enhancements, since IEC certifications in-

volve performance testing, whereas UL certification

is limited to safety-related requirements only.

2.3 Effective Prevention of Quality Devi-ations

This distinctive Quality and Reliability (Q&R) program

has led module suppliers to improve their product

quality when they produce SolarCity modules, which

over time has raised the quality of products deliv-

ered to SolarCity and helped to push excellence in

the entire Solar Industry. The clearly defined system

of controls and tests guarantees that quality is en-

sured from the beginning and sustained over time,

while new quality deviations are rapidly detected.

Monthly ORT testing reveals unforeseen quality

problems. Once problems are detected, we have a

systematic plan in place to implement corrective

actions.

Under this program, SolarCity has implemented the

requirement to perform an end-of-line (EOL) Electro-

luminescence (EL) inspection on 100 % of all mod-

ules fabricated in order to detect defects such as

cracks and micro-cracks across all suppliers. Follow-

ing input from literature and industry collaborations,

guidelines for allowable crack types and size of inac-

tive areas were developed and implemented in or-

der to prevent long-term power loss and risk of

hotspot formation.

Another feature of the Quality Program is that in-

tended changes to the approved BOM require prior

notification and approval by SolarCity, and strict re-

test requirements are in place for any BOM modifi-

cation. SolarCity enforces defined change manage-

ment procedures on each supplier. This feature has

proven successful in many instances. In one case, it

was detected that a supplier had modified the BOM

components without any notification. SolarCity is-

sued a Corrective Action Request (CAR) requiring the

supplier to perform adequate qualification testing

for this modification and any changes thereafter.

Another supplier was found to have modified BOM

components without notification, and the consecu-

tive Corrective Action Request and vendor manage-

ment plan resulted in the implementation of a Glob-

al Change Management program at the supplier in

order to improve oversight and maintain quality.

SolarCity also performs regular factory audits

through internal personnel or independent third-

party auditors to detect issues regarding inconsistent

quality management or quality escapes. As an exam-

ple, inspections at two supplier factories discovered

and corrected a decrease in quality standards before

the product would have been deployed at large scale

in the field. Similarly, factory audits resulted in the

request of improvements to four modules suppliers

to correct deviations with respect to product and

manufacturing quality.

In summary, SolarCity has an unparalleled system in

place to protect quality and prevent the deployment

of faulty modules in the field.

2.4 Constant Refinements and Total In-tegration

The Quality Program is constantly evolving and ex-

panding. Test conditions for initial product qualifica-

tion and ongoing reliability testing are refined on a

regular basis to ensure best possible correlation with

real life performance. As a consequence, the tests

Page 7: SolarCity Photovoltaic Modules with 35 Year Useful Life...SolarCity defines 35 year Useful Life as 95% of mod-ules producing at least 80% of their power after 35 years in their use

7 SolarCity Photovoltaic Modules with 35 Year Useful Life

are becoming more effective and more efficient at

the same time.

The latest revision of SolarCity’s Module Product

Qualification and Ongoing Reliability Test procedures

has been refined and optimized in close collabora-

tion with DNV GL, and DNV GL decided to use it as a

new test standard and implement the program

across all suppliers.

In order to guarantee highest possible quality on a

system level, the Quality Program has been extend-

ed and implemented for key components of the PV

system, such as inverters and connectors. Several

key features of SolarCity’s inverter test program

have also been adopted by DNV GL and implement-

ed in their standard test program.

2.5 World-Class Team behind the Scenes

SolarCity’s Quality team consists of industry-wide

recognized experts on technician-, engineer- and

PhD-level, with extensive experience in relevant in-

dustries such as Solar and Automotive and with

background in fields such as Engineering, Physics,

Chemistry, Materials Science, and Quality Assurance.

The team has repeatedly been recognized by manu-

facturers for their ability to avoid or detect quality

aberrations, as well as to rapidly resolve those by

advising and guiding manufacturers in terms of pro-

cess optimizations and/or materials selection.

3 Useful Life Extrapolation from Accelerated Testing

In the following section, it is demonstrated that So-

larCity’s Total Quality Program is succeeding, and as

a result of the strategic supplier engagement, cou-

pled with rigorous quality requirements, SolarCity

modules show improved performance and projected

lifetime versus other Tier-1 modules tested under

identical conditions in similar timeframes. The sup-

porting data has been generated by accelerated test-

ing within the framework of SolarCity’s PQP and ORT

programs, which was performed by DNV GL PVEL (PV

Evolution Labs), a world-renowned, independent

certified third-party testing laboratory.

3.1 Ongoing Reliability Testing – Over-view

A central part of SolarCity’s Total Quality Program is

Ongoing Reliability Testing. Module manufacturers

are required to submit a defined number of modules

every month, which have randomly been selected

from a typical manufacturing line under supervision,

for reliability testing by an independent third-party,

such as DNV GL. The testing is done according to the

IEC 61215 standard. The overall test duration is

about 16 weeks per batch. An overview of the re-

quired test procedures is listed in Table 2.

Table 2: Overview of ORT test conditions for monthly qual-

ity assurance.

Certifications according to standards such as

IEC 61215 have gained industry-wide acceptance

over the last 15 years. The stress tests defined in the

standards are short-duration accelerated tests per-

formed at stress levels higher than the operating

stress level, so the occurrences of failure modes can

be stimulated within reasonable timeframes. The

qualification tests constitute a minimum require-

ment on reliability testing and are a measure for the

ability of the module to withstand prolonged expo-

sure in real life use environments. It is widely ac-

cepted that these test procedures are appropriate to

identify infancy failures and product weaknesses.

While the tests prescribed in these standards are not

fully adequate to determine the exact working life-

time of a module, the stress conditions prescribed by

these standards are, however, derived from real-life

outdoor stresses. The climate chamber tests yield an

accepted indication of the longevity to be expected,

the quality of the materials, and the workmanship of

the products.

The ORT data shown in the following sections, was

obtained from DNV GL PVEL, SolarCity’s approved

independent third-party testing lab. It summarizes

# Test ORT

1 Initial characterization IV, EL, Visual

2 Thermal Cycling TC-200

3 Damp Heat DH-1000

4 Humidity Freeze TC50/HF10

5 Dynamic Load DML/TC50/HF10

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8 SolarCity Photovoltaic Modules with 35 Year Useful Life

data from 10 monthly batches each consisting of two

modules per test condition from seven SolarCity ap-

proved module manufacturers.

Figure 2: Summary of ORT data obtained from DNV GL

PVEL for accelerated testing on modules from seven Solar-

City approved module manufacturers. Each symbol repre-

sents the average module power degradation for 10

monthly batches each consisting of two modules per test

condition: Thermal Cycling (‘TC’, 200 cycles; green circles),

Damp Heat (‘DH’, 1000 hours; red triangles), combined

Thermal cycling and Humidity Freeze (‘TC+HF’, 50 thermal

cycles followed by 10 humidity freeze cycles; grey dia-

monds), Dynamic Mechanical Load testing followed by

Thermal Cycling and Humidity Freeze (‘DML+TC+HF’, 1000

mechanical load cycles followed by 50 thermal cycles and

10 Humidity Freeze cycles; orange squares).

Figure 2 shows a summary of ORT data for acceler-

ated testing that was performed on modules sup-

plied by seven key SolarCity approved module manu-

facturers. Each symbol represents the average mod-

ule power degradation of 10 monthly batches each

consisting of two modules per test condition. The

median power degradation for all tests and all mod-

ule suppliers combined is as low as -1.1 % (± 0.1 %

standard error) and therefore significantly lower

than the pass criteria of -5 % of IEC 61215. The mod-

ules from all suppliers have a tight distribution

across all test conditions, indicating excellent pro-

cess control and product quality. While still well be-

low the allowed pass criteria, the modules of suppli-

er 7 showed a slightly larger degradation for Thermal

Cycling and mechanical load testing than SolarCity

considers satisfactory, and the supplier was request-

ed to implement a corrective action plan to improve

the reliability performance.

3.2 ORT data – Thermal Cycling

The industry-standard test to simulate thermal

stresses in PV modules as a result of changes of ex-

treme temperatures is Thermal Cycling (TC). PV

modules are fabricated using several materials in-

cluding silicon, metals, polymers, glass, etc. During

temperature changes, these materials expand and

contract according to their coefficient of thermal

expansion (CTE). Therefore, interfaces in the mod-

ules are mechanically stressed due to their differ-

ences in CTE every time a module heats up during

day-night cycles or, what is more, during cycles be-

tween cloud coverage and sunlight. For example,

copper-based ribbons, which electrically connect

neighboring cells in the module, are soldered to the

cells made of silicon, and due to a large difference in

the CTE of metal and silicon, temperature changes

can cause significant mechanical stress to these sol-

der joints. One of the main effects of Thermal Cy-

cling is to simulate the stress on the soldered con-

nections within the module. This may trigger fatigue

of the ribbons, interruption of the electric circuitry,

cell cracking, and power degradation. The modules

are placed in an environmental chamber and sub-

jected to extreme temperature swings from -40 ⁰C to

+85 ⁰C for 200 times, while maximum power current

is sourced into the panel whenever the temperature

exceeds 25 °C. Thermal Cycling is considered a key

accelerated test. Together with Damp Heat testing,

failures due to Thermal Cycling can account for more

than 70% of the total failures for c-Si modules after

accelerated testing.

There is no consensus on the acceleration factor of

this test due to the dependency on environmental

factors, so it is difficult to relate number of cycles to

years in the field. However, the interconnection fail-

ures seen after TC testing are among the most com-

mon failures that are observed in the field. For ex-

ample, long-term studies of modules in the field of

21 manufacturers have shown that of all failures

observed, the highest fraction was due to failed elec-

trical interconnects (as much as 36 %, see Figure 3)

[6].

-6

-5

-4

-3

-2

-1

0

Supplier 1 Supplier 2 Supplier 3 Supplier 4 Supplier 5 Supplier 6 Supplier 7

Po

wer

deg

rad

ati

on

[%

]

ORT Summary

TC

DH

TC+HF

DML+TC+HFIEC Pass Criteria

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9 SolarCity Photovoltaic Modules with 35 Year Useful Life

Figure 3: Field study of PV module failures found for vari-

ous PV modules of 21 manufactures installed in the field

for 8 years. The rate is given relative to the total number

of failures. Approximately 2% of the entire fleet are pre-

dicted to fail after 11-12 years (do not meet the manufac-

turer's warranty). [Taken from Ref 5].

Similarly, a study on returns from a fleet of

>3 million modules from ~20 manufacturers [14]

highlights the significance of Thermal Cycling testing

and the fact that this test is suitable to reveal weak-

nesses of the electrical interconnections. The study

found that the majority (~66%) of modules with in-

fancy failures (returned after an average deployment

of 5 years), were returned because of problems with

electrical interconnections in the laminate (e.g.

breaks in the ribbons and solder bonds).

Figure 4 shows a statistical overview of Thermal Cy-

cling TC-200 ORT test data that was obtained from

DNV GL PVEL, SolarCity’s approved independent

third-party testing lab. The data is a statistically sig-

nificant overview of ~350 modules submitted for

ORT testing performed at DNV GL PVEL. It compares

TC data from ~70 modules fabricated for SolarCity

against data of ~280 Non-SolarCity modules.

Figure 4: Thermal Cycling data comparing the power change after 200 thermal cycles from -40 C to +85 C of ~70 modules

fabricated for SolarCity (‘SCTY’, green bars and line) against data of ~280 Non-SolarCity modules from an industry mix of mod-

ule makers (‘Non-SCTY’, red bars and line). The colored bars represent actual data points, while the lines are Gaussian fits.

0.7% better

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10 SolarCity Photovoltaic Modules with 35 Year Useful Life

The data demonstrates that SolarCity modules show

significantly better performance after Thermal Cy-

cling stress testing than their Non-SolarCity counter-

parts. From the Gaussian fits to the actual data

points, it can be seen that the power degradation for

SolarCity modules peaks at -1.4 %, which is 0.7 %

better than the Non-SolarCity-type modules (-2.1 %),

corresponding to an improvement by almost

one sigma. A statistical hypothesis test confirmed

that the difference in means is statistically significant

(p < 0.001). In addition, SolarCity modules show a

35 % tighter distribution than the Non-SolarCity

modules, and with a maximum degradation of 4 %,

there are no outliers falling beyond the maximum

allowable 5 % threshold. In contrast, as much as 6 %

of all modules not made for SolarCity show degrada-

tion in excess of 5 %, and 3 percent degrade by as

much as 7 %.

The significantly improved reliability performance of

SolarCity modules is attributed to the strict require-

ments that SolarCity imposes on its modules suppli-

ers with respect to all processes that are related to

the electrical interconnections, since this aspect is

considered key to reliable long-term performance as

explained above. For example, SolarCity successfully

imposed 100 % end-of-line electroluminescence (EL)

inspection on all of its suppliers to reliably detect

problems related to soldering of electrical intercon-

nections. During factory audits, SolarCity’s experts

place key focus on inspecting all process steps relat-

ed to the interconnections, and great success has

been achieved in detecting and resolving problems

with these processes.

3.3 ORT data – Damp Heat, Humidity Freeze, and Dynamic Mechanical Load Testing

Another industry-standard test is the Damp Heat

1000 (DH-1000) test, which simulates the effects of

moisture and humidity effects. In this test, the prod-

uct is placed in an environmental chamber at 85 °C

and 85% relative humidity (RH) for 1000 hours. Simi-

lar to other tests within the standard certification

procedures, there is no consensus as to its accelera-

tion factor and the time of exposure in the field it

corresponds to, especially given that there is a

strong influence of the climate zones which the

modules are deployed in; rather, the test is consid-

ered appropriate to exclude short- and near-term

problems and indicate a nominal level of safety in

the field. The test is useful to evaluate the quality of

lamination, which protects the solar cells from hu-

midity ingress. In particular, Damp Heat is a stress

test to evaluate the quality of the encapsulant (mois-

ture protection) and test for any degradation due to

corrosion. Typically, PV module backsheets and en-

capsulants do allow water vapor to pass through,

which may cause stress on interfacial adhesion and

lead to delamination. However, for safe operation,

the interfaces in a PV module must remain adhered

during the entire product lifetime. The main failure

modes triggered by DH testing are backsheet and/or

encapsulant adhesion loss resulting in delamination

and junction box adhesion loss, both of which can

cause safety problems, and other modes are con-

tamination problems, material weaknesses, and

electrochemical corrosion.

In general, the failure rates for Damp Heat testing

appear to have declined during recent years. Nowa-

days, manufacturers have on-site environmental

chambers for the assessment of new products and

materials, which is very effective for failure preven-

tion. Additionally, advances in encapsulation materi-

als and the lamination process, as well as better

edge sealing methods led to an improved protection

against moisture ingress.

The graph in Figure 5 shows a statistical overview of

Damp Heat ORT test data (1000 hours at 85 °C / 85 %

relative humidity) that was obtained from DNV GL

PVEL. The data is a statistically significant overview

of more than 350 modules submitted for ORT testing

performed at DNV GL PVEL. It compares DH data

from ~70 modules supplied to SolarCity against data

of ~280 modules from modules that were not fabri-

cated for SolarCity.

For both SCTY- and Non-SCTY-type modules, the

degradation after Damp Heat testing is low and on-

ly -0.8 % and -0.6 %, respectively. This points to the

fact that the encapsulation materials and lamination

processes are well controlled. However, for all Solar-

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11 SolarCity Photovoltaic Modules with 35 Year Useful Life

City modules the degradation stays below 3 % after

Damp Heat testing, whereas 6 % of Non-SolarCity

modules show degradation between 3 and 5 %. At

the same time, the SolarCity modules have a signifi-

cantly tighter distribution. The standard deviation

for Non-SolarCity modules is with 1.2 % twice as high

as for SolarCity modules (0.6 %), and a statistical

hypothesis test confirms that the difference is statis-

tically significant (p < 0.001). These observations

indicate a further improvement in process and quali-

ty control for the SolarCity modules and an effective

prevention of problems due to moisture ingress.

Figure 5: Damp Heat data comparing the power change

(bottom graph) after 1000 hours of Damp Heat at 85 C

and 85 % relative humidity (RH) of ~70 modules fabricated

for SolarCity (‘SCTY’, green bars and line) against data of

~280 Non-SolarCity modules from an industry mix of mod-

ule makers (‘Non-SCTY’, red bars and line). The colored

bars represent actual data points, while the lines are

Gaussian fits. The top graph shows the standard deviations

for both types of modules.

As described above, PV modules are not impermea-

ble to water vapor, which can lead to a weakening of

interfacial adhesion over time. When moisture pre-

sent inside of the laminate freezes, ice crystals may

cause additional damage to the interfaces in the

module and cause delamination. The Humidity

Freeze (HF) test is an environmental test designed to

determine the module's ability to withstand the ef-

fects of high temperatures combined with humidity,

followed by extremely low temperatures. PV mod-

ules are subjected to temperatures of 85°C and rela-

tive humidity of 85 % for 21 hours, which causes

partial saturation of the module with water. The

modules are then cooled down to -40 °C, which

causes the moisture to freeze. The modules are sub-

jected to 10 complete cycles in the closed climatic

chamber.

Figure 6: ORT data comparing the power change after

combined 50 cycles of Thermal Cycling and 10 Humidity

Freeze cycles (top), and Dynamic Mechanical Load testing

followed by 50 cycles of Thermal Cycling and 10 Humidity

Freeze cycles (bottom) of ~200 modules of SolarCity mod-

ule suppliers against data of ~200 modules from industry

mix of module makers that are not suppliers to SolarCity.

Symbols are actual data points, while the lines are Gaussi-

an fits with the following fit parameters: a is the height of

the curve's peak, b is the position of the center of the

peak, and c (Gaussian RMS width) controls the width of

the "bell".

20.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

5- 4- 3- 2- 1- 0 1

-0.8333 0.6068 67

-0.5644 1.204 281

Mean StDev N

P

sel

ud

om ll

a fo

noit

car

F

)%( noitadargeD rewo

S

elbairaV

YTCS-noN

YTC

H lamroN

YTCS-noN ,xamP atleD ,YTCS ,xamP atleD fo margotsi

0%

5%

10%

15%

-5 -4 -3 -2 -1 0 1 2

Fra

cti

on

of

all

mo

du

les

(%

)

Power degradation (%)

DH1000

SCTY

Non-SCTY

SCTY

Non-SCTY

Gaussian Fit Non-SCTY SCTY

a 0.1 0.1

b -0.3 -0.5

c 1.2 -0.7

0%

10%

20%

30%

40%

50%

-5 -4 -3 -2 -1 0 1 2

Fra

cti

on

of

all

mo

du

les

(%

)

Power degradation (%)

TC50-HF10

SCTY

Non-SCTY

SCTY

Non-SCTY

Gaussian Fit Non-SCTY SCTY

a 0.2 0.2

b -1.3 -1.1

c -1.2 0.9

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

-7 -6 -5 -4 -3 -2 -1 0 1 2

Fra

cti

on

of

all

mo

du

les

(%

)

Power degradation (%)

DML

SCTY

Non-SCTY

SCTY

Non-SCTY

Gaussian Fit Non-SCTY SCTY

a 0.1 0.4

b -3.2 0.0

c 4.0 1.2

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12 SolarCity Photovoltaic Modules with 35 Year Useful Life

The main failure modes triggered by Humidity Freeze

testing are caused by thermo-mechanical stress due

to the different thermal coefficients of glass, Silicon,

and copper, and consist of delamination, junction

box detachment, and/or cell interconnect failures.

Further, high-temperature glass corrosion can occur

as a result of alkali removal from the glass surface.

The freezing of the moisture propagates the corro-

sion effect deeper into the glass. A porous silica ma-

terial may form as a result of glass corrosion, which

affects the transmission properties of the glass and

can cause reduced module power output.

From Figure 6 (top), it can be seen that for both

SCTY- and Non-SCTY-type modules, the degradation

after 10 cycles of Humidity Freeze testing is on-

ly -1.3 % and -1.1 %, respectively. This confirms that

the encapsulation materials and lamination process-

es are well controlled, resulting in a high-quality lam-

inate. The degradation in SCTY-type modules is 0.2 %

lower than in Non-SCTY-type modules, and similar to

Damp Heat testing, the distribution is tighter. The

data for SolarCity modules are closer to Gaussian,

while the non-SCTY data deviate from a Gaussian

shape and reflect an inhomogeneous distribution,

confirming the improved process and quality control

for the SolarCity-type modules that was suggested

from the Damp Heat results and indicating the effec-

tive prevention of problems due to moisture ingress

and laminate deficiencies.

Dynamic Mechanical Load (DML) testing will be dis-

cussed next. Modules in the field are subjected to

mechanical stress due to wind and snow loads. The

resulting deflections depend on glass thickness, en-

capsulant and backsheet properties, frame design, as

well as temperature and magnitude of the loads.

Cycling deflections may result in the formation of

cell cracks. In order to simulate the stress caused by

wind and snow loads, the modules are first subject-

ed to 1,000 Dynamic Mechanical Load cycles to test

for the formation of cracks. However, the output

power often stays unaffected unless the crack fully

penetrates the metallization on the rear side of the

cell. Therefore, the modules are also exposed to 50

Thermal Cycles, which can cause the propagation of

cracks that may have formed. Lastly, the modules

need to undergo ten Humidity Freeze cycles. The

high humidity followed by freezing temperatures

causes the cracks to propagate through the cell met-

allization. Failures that are seen after DML testing

are broken glass, cracked cells, and/or damaged

electrical interconnect ribbons.

The results from Dynamic Mechanical Load testing

on the 400 modules submitted for ORT testing to

DNV GL PVEL are plotted in the bottom graph of Fig-

ure 6. A clear advantage of SolarCity-type modules is

evident. After the DML stress testing, the distribu-

tion for the degradation of Non-SolarCity-type mod-

ules has a maximum at -3.2 %, whereas this value for

the SolarCity-type modules is at 0 %. Additionally,

the distribution is noticeably tighter for SCTY-type

modules with an RMS value of 1.2 compared to 4.0.

The excellent mechanical stability confirms the re-

sults from above and demonstrates the high quality

of SCTY-type modules and their integrity against

thermo-mechanical stresses.

In Figure 7, electroluminescence (EL) images of a

representative SCTY module supplier before DML

testing (bottom left), after 1,000 cycles of Dynamic

Mechanical Load testing (bottom center), and after

the completed DML test sequence DML/TC/HF (bot-

tom right) are shown. The data verifies the mechani-

cal integrity of SCTY-type modules. Even after this

aggressive test procedure, the power degradation is

as low as -0.8 %, demonstrating the improved me-

chanical stability of SolarCity’s patented Zep Solar

Panel Mounting System compared to the conven-

tional mounting system of Non-SolarCity modules.

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13 SolarCity Photovoltaic Modules with 35 Year Useful Life

Figure 7: Electroluminescence images and corresponding power degradation of a representative SCTY module supplier before

DML testing (left), after 1,000 cycles of Dynamic Mechanical Load testing (center), and after the completed DML test sequence

of 1,000 Dynamic Mechanical Load cycles, 50 Thermal Cycles, and 10 Humidity Freeze cycles (right).

3.4 PQP Testing – Testing Beyond Stand-ard Qualification Tests

The ORT test sequence described above follows the

IEC 61215 test standard. It is generally understood

that these qualification tests are a minimum re-

quirement of reliability tests and appropriate to de-

tect infant mortality failures and anticipate short-

term reliability issues in the field. Passing these

standard tests demonstrate the ability of modules to

withstand prolonged exposure in general use envi-

ronments. However, there is a gap with respect to

long-term performance prediction, and there is

broad consensus in the solar community that the IEC

standards allow no conclusions to be made concern-

ing the actual lifetime expectancy for tested prod-

ucts. There is agreement that lifetime depends on

the design, the materials, the manufacturing quality,

and the use environment under which the product is

operated.

To address the gap between the standard IEC qualifi-

cation tests and long-term performance prediction,

several global standards development activities are

underway, which are primarily based on extending

the individual tests of IEC 61215. SolarCity follows

this methodology and implemented a Product Quali-

fication Program testing sequence that is based on

extended IEC 61215 tests, with an added electrically

biased Damp Heat test to evaluate Potential In-

duced Degradation (PID) and a sequence of dynamic

mechanical load testing followed by Thermal Cycling

and humidity freeze. While adding significant confi-

dence that the more demanding test procedures

allow to better predict the long-term reliability of

the tested modules, there is still no clear under-

standing of whether the expanded tests trigger real-

istic failures or instead failures that are not found

under realistic operating conditions. Therefore, So-

larCity has a program underway to evaluate the va-

lidity of the test results of these extended tests and

correlate these to the performance in the field under

real life conditions. SolarCity has a unique advantage

of having direct access to one of the largest net-

works of installed residential and commercial PV

systems in a large variety of use environments. Deg-

radation rates under real-life conditions will be eval-

Pre stress DML-1000 DML-1000 / TC50 / HF10

Pmax change

( rel. %)0 -0.9% -0.8%

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14 SolarCity Photovoltaic Modules with 35 Year Useful Life

uated and failure modes observed will be correlated

to the ones seen in accelerated testing.

3.5 Product Qualification Program Test-ing – Overview

Besides ORT testing discussed above, a second key

part of SolarCity’s Total Quality Program is the Prod-

uct Qualification Program Testing. The tests are

based on extended IEC 61215 tests, with an added

electrically biased Damp Heat test to evaluate Po-

tential Induced Degradation (PID) as well as a se-

quence of dynamic mechanical load testing followed

by Thermal Cycling and humidity freeze testing.

While not yielding conclusive information about the

product life expectancy, the successful passing of

such tests still justify the assumption of longer life-

times. SolarCity believes that the extended upfront

testing required for product qualification in conjunc-

tion with the continuous Ongoing Reliability Testing

in regular intervals, constitute a quality program that

is industry-leading and appropriate to justify module

Useful Life of 35 years.

Table 3: Overview of PQP test conditions for qualification

of initial products or modified BOMs.

For initial product qualification and after any chang-

es to the Bill of Materials, module manufacturers are

required to submit a defined number of modules,

which have randomly been selected from a typical

manufacturing line under supervision, for reliability

testing by an independent third-party, such as DNV

GL PVEL. An overview of the required test proce-

dures is listed in Table 3.

3.6 Product Qualification Program Test-ing – Extended Thermal Cycling

As mentioned above, the tests that show the largest

effect on PV module performance and appearance

are Temperature Cycling tests as well as Damp Heat

testing. It has been shown that Thermal Cycling with

injected current is an appropriate test to reveal de-

sign weaknesses and identify early failures of cell

interconnect ribbons and solder bonds [15, 16].

However, it is generally understood that the 200

thermal cycles from IEC 61215 testing are not suffi-

cient to give confidence in a module lifetime of more

than 25 years [17, 18]. While there is evidence that

longer Thermal Cycling is a more adequate test to

ensure long-term reliability and reduce field failures,

there is no consensus as to how many cycles corre-

spond to what lifetime, especially given that there

may be significant variations with climate and use

conditions.

Studies on an extended number of modules have

shown that with an increasing number of thermal

cycles beyond the standard of 200 cycles, problems

with electrical interconnects between the cells can

occur, as is evident from dark areas in electrolumi-

nescence images. In general, the degree of damage

will get more severe and the output power will de-

crease with increasing number of cycles. The in-

creased number of cells with broken busbars can

lead to an inhomogeneous current distribution be-

tween the cells. This can lead to serious safety prob-

lems, since high temperatures or even hot spots and

arcing can occur.

In Figure 8, the relative output power after a con-

secutive sequence of 200, 400, 600, and 800 Ther-

mal Cycles is plotted for a representative mix of So-

larCity-type modules (green line with filled circles;

median and standard error) and compared against a

statistically significant mix of Non-SolarCity-type

modules (66 %ile: black line; 50 %ile: grey line;

33 %ile: light grey line) that have been measured at

DNV GL PVEL over time. The modules discussed here

are the collection of modules that were submitted

by module suppliers to DNV GL for PQP testing,

where ‘SolarCity-type’ modules were fabricated for

SolarCity, while ‘Non-SolarCity-type’ modules made

# Test PQP

1 Thermal Cycling TC-800

2 Damp Heat DH-3000

3 Humidity Freeze TC50/HF10 (3x)

4 UV Exposure 90 kWh

5Dynamic Mechanical Load /

Thermal Cycling/ Hum. Freeze

1000 cycles (1440 Pa) /

TC50 / HF10

6 PID testingBoth Polarities;

600 hours @ 85C/85%

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15 SolarCity Photovoltaic Modules with 35 Year Useful Life

up the rest of the collection and are referred to as

‘Industry mix’. Additionally, data from a selection of

modules from a comprehensive literature study is

shown (red dashed lines) [14]. As can be seen from

the plot, the median degradation for a representa-

tive mix of modules from SolarCity suppliers after

this extensive stress test is as low as 2 %. The com-

parison with a large number of Non-SolarCity-type

modules demonstrates that SolarCity-type modules

perform at least as well as the very best modules in

industry and outperform the vast majority of their

industry counterparts. After 800 cycles, DNV GL

PVEL’s 66 percentile for the average degradation is

1 % higher.

Figure 8: Relative output power after a consecutive series of 200, 400, 600, and 800 Thermal Cycles from -40 °C to +85 °C for a

representative mix of SolarCity-type modules (green line with filled circles; median and standard error) and a statistically signifi-

cant mix of Non-SolarCity-type modules (66 %ile: black line; 50 %ile: grey line; 33 %ile: light grey line) that have been measured

at DNV GL PVEL over time. Additionally, data from a comprehensive literature study is shown (red dashed lines) [14].

This important result further strengthens the find-

ings from section 3.2. Even when subjected to one of

the most aggressive test procedures currently used

in industry, SolarCity-type modules are resilient to

metallization-related problems and do not show any

gridline interruptions or problems with the electrical

interconnects, which have been shown to be one of

the most prevalent failure modes that have been

observed in modules in the field [6].

Figure 9 visualizes this finding. Electroluminescence

images and corresponding power degradation of

modules from a representative SCTY supplier before

Thermal Cycling testing (left), after 400 Thermal cy-

cles (center), and after 800 Thermal cycles (right). It

can be seen that the electrical interconnects and

gridlines are intact, and the degradation after 800

cycles is as low as -1.3 %. For comparison, EL images

of modules from the literature [14] after 200 ther-

mal cycles (left), 400 thermal cycles (center), and

after 600 thermal cycles (right) are shown in Figure

10, and dark areas appearing after >400 cycles indi-

cate disconnection of busbars (red markers).

The insignificant degradation after extended TC is

credited to the strict requirements that SolarCity

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16 SolarCity Photovoltaic Modules with 35 Year Useful Life

imposes on its modules suppliers with respect to all

processes that are related to the electrical intercon-

nections. As mentioned above, SolarCity successfully

imposed 100 % end-of-line electroluminescence (EL)

inspection on all of its suppliers to reliably detect

problems related to soldering of electrical intercon-

nections. During factory audits, SolarCity’s experts

place key focus on inspecting all process steps relat-

ed to the interconnections, and great success has

been achieved in detecting and helping resolve prob-

lems with these processes.

However, as discussed above, the correlation of de-

fects seen after such extended TC tests and failures

occurring in the field has not unambiguously been

proven, and it still a matter of debate whether these

extensive stress tests might overstress the modules

and trigger issues that would not occur in the same

way in the field.

Nonetheless, the fact that after accelerated TC test-

ing problems commonly affecting a large fraction of

modules after extended amounts of time in the field

are not observed, justifies the assumption that the

degradation rate of SolarCity modules will be at least

as low as the industry average for modules of 0.5 %

per year [1] if not better, and a Useful Life of

35 years yielding a power output of 82.5 % thereaf-

ter appears realistic.

Figure 9: Electroluminescence images and corresponding power degradation of modules from a representative SCTY supplier

before Thermal Cycling testing (left), after 400 Thermal cycles (center), and after 800 Thermal cycles (right).

Pre stress TC-400 TC-800

Pmax change

( rel. %)0 -0.9% -1.3%

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17 SolarCity Photovoltaic Modules with 35 Year Useful Life

Figure 10: Electroluminescence images of a modules after 200 Thermal cycles (left), 400 Thermal cycles (center),

and after 600 Thermal cycles (right). Dark areas indicate disconnection of busbars (red markers) [14].

3.7 Product Qualification Program Test-ing – Extended Damp Heat

The second most common test to demonstrate long-

term PV module performance is extended Damp

Heat testing. Figure 11 shows the change of output

power after Damp Heat testing for 1000, 2000, and

3000 hours for a mix of SolarCity-type modules

(green line with filled circles; median and standard

error) and a statistically significant mix of Non-

SolarCity-type modules (66 %ile: black line; 50 %ile:

grey line; 33 %ile: light grey line) that have been

measured at DNV GL PVEL over time. The modules

discussed here are the collection of modules that

were submitted by module suppliers to DNV GL for

PQP testing, where ‘SolarCity-type’ modules were

fabricated for SolarCity, while ‘Non-SolarCity-type’

modules made up the rest of the collection and are

referred to as ‘Industry mix’. Additionally, data from

a selection of modules of a comprehensive literature

study is shown [14].

Even after exposure to 3000 hours of Damp Heat,

which is three times the time required by IEC 61215,

the SolarCity-type modules only show ~2 % median

power degradation.

The extension of Damp Heat testing to 2000 or even

3000 hours has become common practice in the at-

tempt to demonstrate greater durability of a particu-

lar module design, and SolarCity requires this test as

standard for the Product Qualification Program.

However, 3000 hours of Damp Heat is considered a

test to failure, and it has increasingly been reported

that the problems observed in this test are not rep-

resentative of failures occurring in the field [14]. It is

under debate whether the 3000 hour Damp Heat

test performed on a module with a breathable back-

sheet is useful for the prediction of service life in the

field. However, the fact that the SolarCity-type mod-

ules do not show significant degradation after this

extended test suggests excellent protection against

long-term effects caused by excess humidity and/or

heat. In order to address possible limitations of ul-

tra-long Damp Heat testing, SolarCity will perform

additional UV exposure tests, as described in sec-

tion 4.

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18 SolarCity Photovoltaic Modules with 35 Year Useful Life

Figure 11: Power degradation after Damp Heat testing for 1000, 2000, and 3000 hours for a mix of SolarCity-type modules

(green line with filled circles; median and standard error) and a statistically significant mix of Non-SolarCity-type modules

(66 %ile: black line; 50 %ile: grey line; 33 %ile: light grey line) that have been measured at DNV GL PVEL over time. Additionally,

data from a comprehensive literature study is shown [14].

3.8 Product Qualification Program Test-ing – PID Testing

Another extended stress test that SolarCity requires

within the framework of its Product Qualification

Program is a test for Potential Induced Degradation

(PID). In the field, the voltage of modules that are

connected in series within strings commonly reaches

-600 V or -1000 V. Per US code, the module frames

need to be grounded, which causes a voltage differ-

ence between the grounded frames and the cells in

the module. This voltage can cause a migration of

mobile ions through the module either towards or

away from the cells. As a result, mobile positive so-

dium ions contained in the glass substrate can mi-

grate towards the cells. Crystalline defects known as

stacking faults with lengths of just a few microme-

ters permit the ingression of these sodium atoms,

which results in short circuits (shunts) that are symp-

tomatic of the Potential Induced degradation. The

effect is triggered by humidity, temperature, and

voltage. SolarCity implemented one of the most ag-

gressive procedures in the industry to test for PID.

The modules are subjected to 85 °C and 85 % RH,

while simultaneously biased with positive or nega-

tive 1000 V with respect to the module frame. The

degradation is tested after 100 and 600 hours expo-

sure, respectively. As a comparison, NREL’s ad-

vanced ‘Qualification Plus’ standard that was de-

signed to address the shortcomings of IEC 61215

only postulates a 96 hours long exposure to 60 °C

and 85 % RH and -1000 V.

Figure 12 shows a plot of the power degradation

after PID testing at for 100 and 600 hours for a mix

of SolarCity-type modules (green line with filled cir-

cles; median and standard error) and a statistically

significant mix of Non-SolarCity-type modules

(66 %ile: black line; 50 %ile: grey line; 33 %ile: light

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19 SolarCity Photovoltaic Modules with 35 Year Useful Life

grey line) that have been measured at DNV GL PVEL

over time. The modules discussed here are the col-

lection of modules that were submitted by module

suppliers to DNV GL for PQP testing, where ‘SolarCi-

ty-type’ modules were fabricated for SolarCity, while

‘Non-SolarCity-type’ modules made up the rest of

the collection and are referred to as ‘Industry mix’.

Similar to the extended Thermal Cycling and Damp

Heat tests discussed above, SolarCity-type modules

outperform the majority of their counterparts in

industry. Even after 600 hours of aggressive PID ex-

posure at 85 °C and 85 % RH and applied bias

of -1000 V, the output power degrades less than 1 %

and is less than the degradation seen in the best

66 percentile of Non-SolarCity-type modules tested

at DNV GL PVEL. SolarCity’s modules are considered

‘PID-free’, even under the most aggressive operating

conditions currently tested for in the industry.

Figure 12: Power degradation after PID testing at 85 °C /

85%RH and -1000V for 100 and 600 hours for a mix of

SolarCity-type modules (green line with filled circles; me-

dian and standard error) and a statistically significant mix

of Non-SolarCity-type modules (66 %ile: black line; 50 %ile:

grey line; 33 %ile: light grey line) that have been measured

at DNV GL PVEL over time.

3.9 Product Qualification Program Test-ing – Extended Humidity Freeze and UV Test

Similar findings to the ones described in sections 3.6

to 3.8 were observed for the remaining tests of So-

larCity’s PQP program that are listed in Table 3.

A representative mix of SolarCity-type modules show

less than 2 % power degradation (not shown here)

after extended Humidity Freeze testing (3 times the

duration of IEC 61215 testing that is described in

section 3.3). This result confirms the findings from

extended Damp Heat testing and shows that the

encapsulation materials and lamination processes

are well controlled, resulting in a high-quality lami-

nate and an effective prevention of problems due to

moisture ingress and laminate deficiencies.

Similarly, the power degradation observed for a rep-

resentative mix of SolarCity-type modules is ~1 %

after exposure to UV radiation at 60 °C for a total

exposure of 90 kWh/m2 (not shown here), indicating

that these modules do not show any UV-induced

optical or mechanical degradation under the condi-

tions tested.

4 Next Steps – Tests with Im-proved Correlation to Real Life

As discussed above, there is no broad consensus as

to how well current extended accelerating test pro-

cedures mimic the stress conditions that modules

experience under real-life conditions and how the

failure modes triggered by these extreme stress tests

match defects seen in real modules. It is not clear

whether the extended conditions lead to overstress-

ing of the modules and to failure modes that are not

observed in the field under real life conditions. At

the same time, defects and failures that are seen in

modules in the field may not be detected by the cur-

rent accelerated test procedures.

For example, there has been substantial discussion

of a general need for longer UV exposure [19], given

that the UV exposure in current stress tests is orders

of magnitude weaker than the expected UV dosage

that modules experience during their life in the field.

The UV preconditioning test according to IEC 61215

with a UV dosage of 15 kWh/m2 between 280 and

385 nm only simulates about 18 days of AM1.5 ir-

radation at 1000 W/m2 [20]. Even for an assumed

typical outdoor day/night average of 250 W/m2 for

outdoor irradiation, the test still only simulates

~2.4 months of outdoor irradiation. Similarly, there

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20 SolarCity Photovoltaic Modules with 35 Year Useful Life

are currently no standardized test procedures to

detect the formation of snail trails, which have be-

come a widespread phenomenon that is encoun-

tered by a large number of module makers and solar

farms across the world.

SolarCity considers itself a thought leader in the in-

dustry and is an active player in the development

and implementation of more advanced accelerated

testing procedures. These tests address some of the

serious shortcomings of current accelerated stress

tests and help generate new relevant and meaning-

ful results. SolarCity has been working with leading

institutions and industry partners such as NREL, DNV

GL PVEL, and DuPont to detect and address serious

and common failure modes occurring in modules

after exposure to real life use conditions in order to

ensure that its modules are robust against the

mechanisms causing these real life failures.

In view of this, novel tests have been developed and

implemented for detection of snail trails, hotspots,

advanced UV degradation of backsheets, and degra-

dation of mechanical properties of backsheets. So-

larCity also introduced a field degradation study

where modules are placed outdoor and character-

ized after six and twelve months to evaluate the

degradation rates under real-life conditions. Last,

more sensitive methods to allow for early detection

of defects related to increased series resistance have

been incorporated and are under evaluation.

All these tests have been implemented as an addi-

tion to SolarCity’s PQP product qualification test plan

in February 2016, and they are now a mandatory

requirement for product qualification for all SolarCi-

ty 3rd

party module suppliers.

The development of the advanced test procedures

was led by SolarCity engineers, in collaboration with

engineers from DNV GL PVEL. Approving of these

advanced test methods, DNV GL PVEL implemented

the same test procedures as a new standard for their

Product Qualification plan.

5 Useful Life Extrapolation Based on Degradation of Fielded Mod-ules

Accelerated testing is a valuable tool to uncover ear-

ly product failures and indicate reliable long-term

performance, but as mentioned above, it is challeng-

ing to obtain quantitative information about degra-

dation rates under realistic use conditions. A real-

world environment is a unique combination of dif-

ferent stressors, which no accelerated testing cham-

ber is able to accurately duplicate. Such stressors in

include, but are not limited to high and low temper-

atures, rain and moisture, UV irradiance, snow, salt

fog, and soiling. Therefore, outdoor testing under

realistic exposure conditions is the most appropriate

method to correlate indoor accelerated testing to

real-world long-term performance.

SolarCity is in the unique position to have access to

and gain insight from a very large fleet of modules

and systems. The company is currently working on

determining real-life module degradation rates

based on studies of modules that have been de-

ployed in the field in order to support the argument

of 35-Year Useful Life for their modules. The work is

based on a three-pronged approach:

1. Sample representative modules that have been

deployed in the field for several years and test

them in the laboratory to compare against

nameplate rating, following D. Jordan’s et al.

methodology [1,2]. As discussed in the refer-

enced publication, the accuracy of this method is

affected by the uncertainty of nameplate rating

and possible light-induced degradation (LID).

2. Estimate the annual degradation rate from sys-

tem performance data that is collected through

SolarCity (correct the data using weather infor-

mation). This method has a larger error due to

limited accuracy of online performance data, cli-

mate data, impact of soiling, etc.

3. Deploy modules in the field (in various climates)

after thorough characterization in the laboratory

and light exposure to eliminate the effects of LID,

and test them in the laboratory in regular inter-

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21 SolarCity Photovoltaic Modules with 35 Year Useful Life

vals. This method will yield the best possible in-

formation on degradation rates, since the meas-

urements are performed under controlled labor-

atory conditions, and accurate information about

initial performance is available.

The goal of this study is to demonstrate that SolarCi-

ty modules deployed in the field under real-life con-

ditions show an annual module degradation rate of

no more than 0.5-0.6 % per year, which warrants a

postulation of Useful Life of 35 years with a power

output of 80 to 82.5 % thereafter.

This data will be presented in a follow-up publica-tion.

6 Conclusion

SolarCity presented data and supporting information

to support the claim that the Useful Life of the pho-

tovoltaic (PV) modules used in its residential and

commercial systems is 35 years or longer. Data from

accelerated stress testing according to and beyond

IEC 61215, which was performed by DNV GL,

demonstrates that power degradation of modules

supplied to SolarCity by external suppliers is as much

as 35 % lower than for a comparable industry-wide

selection of non-SolarCity modules measured at

DNV GL, which are typically warranted for 25 years.

The reason for this advantage of modules fabricated

for SolarCity is the implementation of a stringent and

industry-leading Total Quality Program, which

adopted its features from the Automotive Industry

and was implemented by SolarCity in early 2014. Per

contractual requirement in Master Purchasing

Agreements, SolarCity’s third-party suppliers need to

have effective Quality Assurance programs and re-

fined manufacturing processes in place, and steady

product and manufacturing quality must be demon-

strated. Rigorous tests need to be passed on an on-

going basis, performed by a qualified 3rd

party lab.

The data presented supports the assertion that So-

larCity’s PV modules, as a result of its comprehensive

Total Quality Program and industry-wide advance-

ments in material, manufacturing, and quality con-

trol, perform at least similar, if not better than the

median crystalline-Si modules observed in the larg-

est meta-study to date of more than 11,000 modules

[1,2]. Therefore, an annual module degradation rate

of 0.5-0.6 % per year is a realistic assumption, which

warrants a postulation of Useful Life of 35 years with

a power output of 80 to 82.5 % thereafter.

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