SolarTester uses sample tests for the testing of large batches of solar panels. By making clear agreements beforehand, we prevent arguments afterwards. Here you can read more about determining the sample test, what the possible escalation path is, and in which situations a batch is accepted or rejected. Determining the sample size and acceptance of the panels is done according to the ISO 2859-1 method. By using this widely recognised and commonly used method, it is clear to everyone what is being tested and how. This scientific substantiation offers clarity regarding the validity of the test results in advance.

Determining the Sample Size

Two aspects play a role in determining the sample size. First there’s the size of the total population. For the tests that SolarTester performs, this refers to the number of panels that will be used for a certain project. The second aspect that is included is the level of inspection. The ISO method lists three levels of testing.

Level 1 Reduced inspection level
This level is used when multiple level 2 tests have been performed successfully for the supplier or production process in the past, or if there is a lot of confidence regarding the product’s quality.
Level 2 Normal inspection level
Level 3 Tightened inspection level
This level is used when there have been past issues with the supplier or the products that have to be tested.

The level is determined per test and can even be changed during testing. For instance, when a level 1 shows more defects than permitted, the inspection level can be increased to 2 or even 3. By using ISO 2859-1, this escalation path is clear in advance.

With the combination of population size and inspection level, a sample size can then be determined from the sample test table. An example of the sample test table is shown below. First we look for the line in the table that includes our population. Next we match it with the column showing the inspection level. The letter in that cell is used to read the sample size elsewhere in the table.

Lot size Level I Level 2 Level 3 Code Size
2-8 A A B A 2
9-15 A B C B 3
16-25 B C D C 5
26-50 C D E D 8
51-90 C E F E 13
91-150 D F G F 20
151-280 E G H G 32
281-500 F H J H 50
501-1200 G J K J 80
1201-3200 H K L K 125
3201-10000 J L M L 200
10001-35000 K M N M 315
35001-150000 L N P N 500
P 800
Q 1250
R 2000

Sample Test Table

Acceptance and Rejection of Solar Panels

In addition to sample size, we have to determine the criteria for acceptance and rejection. When evaluating the quality of products, defects are divided into three categories.

Minor defects: faults that most users are unlikely to notice or mind.
Major defects: faults that a user will not tolerate.
Critical defects: completely unacceptable faults

SolarTester makes use of the MBJ assessment criteria – MBJ is the supplier of our testing lab – and the categorisation above is in line with those criteria. In these, modules are assessed and divided into class A, B, C, or D. A module in class B contains one or more minor defects, class C shows major defects, and class D contains critical defects.

AQL
We use the AQL method for the acceptance and rejection of a batch. AQL is an abbreviation that stands for Acceptable Quality Limit. With the AQL, we determine the maximum percentage of the total population that can show minor, major, or critical defects. AQL 4.0, for example, means that up to 4% of the entire population of projects may have a minor defect.

That means that for a test, we have to determine an AQL for each of the three types of defects: one AQL for minor defects, one for major defects, and one for critical defects. For minor defects, we usually use an AQL 4.0. Because of the fact that there is always some uncertainty about how representative the entire test is for the entire population, a higher percentage is accepted in the sample test. The number of allowed defects per AQL level can be seen in the AQL table. Below you can find an example of a simplified AQL table showing the relevant numbers for 2 AQL numbers, for which the sample test is borderline accepted.

An Example of Determining Sampling

In order to illustrate this topic, we’ve composed an example below. The panels in the example have to be tested for a 3,000-panel project. Because we don’t have a lot of experience with the supplier in this example, we choose level-2 testing. In line with common industry practices, we use AQL 4.0 for minor defects and an AQL of 0.4 for major defects. We employ an AQL 0 for critical defects.

Code Size AQL 0.4 AQL 4.0
A 2 0 0
B 3 0 0
C 5 0 0
D 8 0 0
E 13 0 1
F 20 0 2
G 32 0 3
H 50 0 5
J 80 0 7
K 125 1 10
L 200 2 14
M 315 3 21
N 500 5 21
P 800 7 21
Q 1250 10 21
R 2000 14 21

Simplified AQL table

In the sample testing table, we can see that a 3,000 panel project and a level-2 sample test correspond to the letter K. The simplified AQL table above shows that the letter K corresponds to 125 items, and that at AQL 4, 10 modules can show minor defects, and for AQL 0.4, 1 module can show major defects. Translated to the MBJ criteria, this means that up to 10 modules may be assessed as B, and only 1 module may be assessed as C. None of the modules may be assessed as D.

If it is found during the test that the maximum number of models according to the AQL is reached, it may be decided to increase the sample size. In this example we would scale up from level 2 to level 3. We will then move from letter K to the letter L; the sample size will be expanded from 125 modules to 200. The AQL 4.0 will now be 14, the AQL 0.4 will be 2.

Would you like to learn more about our testing method? Please feel free to contact us.