“The modules all met the same basic safety requirement yet performance was different from one manufacturer to another," according to Intertek Group testing services Regional VP Sunny Rai.

Performance stakes are high, Rai said, so “what banks, developers and manufacturers want to prove is very similar.” Manufacturers want to prove their product provides the promised return on investment (ROI). Banks and developers want to verify they will get the promised ROI.

To do this, Intertek developed its bankability testing. “Bankability is all about performance and ROI,” Rai said.

“The National Electrical Code requires products to be certified safe by a nationally recognized testing laboratory,” Rai said. “UL 1703 is the U.S. safety standard for solar modules.”

Safety testing of a solar module has two parts, Rai said: construction evaluation and actual testing.

“Construction evaluation verifies the product is built to meet the standard, including evaluation of its components, the junction box, back sheet, cells, grounding methods, connectors and wires.” It also includes making certain the components are properly certified and are used properly, secured in place and spaced properly, properly labeled, and so on.

“Once we finish the construction evaluation,” Rai said, “we come up with a plan to verify the module’s compliance with the testing part of the standard."

Tests of different solar modules can vary slightly but are similar “because solar modules are used in similar places and face the same things, like light and rain and wind and snow,” Rai said.


“Because they are expected to be used outdoors for twenty to 25 years, they are subjected to certain tests that we refer to as accelerated aging tests. This is done in conditioning chambers,” Rai said. A set of modules are subjected to “standard test conditions (STC) of 25 degrees C and a perfect one sun light exposure, which is the equivalent of 1,000 watts per square meter of sunlight exposure, from a solar simulator.”

The tested power output after that, Rai said, becomes that module’s rated output.

The set of modules is then divided into three groups. One is “put through the accelerated aging test with temperatures from minus 40 degrees C to plus 85 degrees C in a six-hour cycle over 1,000 cycles.”

Another set is subjected to similar temperatures and a similar rate of change but with 85 percent humidity, “so you are not just heating and cooling it, you are freezing the humidity onto the module, thawing it, heating it, and cooling it back down.”

A third test, not required by the UL standard but required by the IEC standard, Rai said, “is what we call a damp heat test. The module is under heat of 85 degrees C with 85 percent humidity for 1,000 hours non-stop -- a constant damp heat.”

The safety standards don’t require monitoring the modules’ output during these procedures.

“This is where we transition from a safety test to a bankability test. We are monitoring the modules to see how the degradation caused by the temperature and the humidity impacts performance,” Rai explained.

The tests are the same, but output is monitored more closely. “For bankability testing, we don’t necessarily only test for 1,000 hours. We test for 200 hours, pull it out, see what the output degradation is, put it back for another 200 hours and, in some cases, we continue until the output of the module drops below a certain level pre-agreed with the manufacturer or whoever asks for the test.”

Testing can last for up to 2,000 hours. “We have observed that a product’s degradation is accelerated in the first 500 to 600 hours,” Rai said. After that, “the degradation is very slow. In most cases, we stop between 1,500 hours and 2,000 hours.”

The correlation between the number of hours tested and the years on a roof “has still not been done,” Rai said. “NREL has a PV Quality Assurance Task Force that is looking at that. There are not yet enough installed years to establish that correlation.”

Intertek’s bankability testing can include other procedures or time frames. “It is targeted to a specific type of installation and modified based on what we are trying to prove or determine. Some buyers or manufacturers want to create a worst-case scenario.”

Each test, Rai said, takes "from 40 to 45 days to complete. You are looking at over six months for testing and, in between, you are doing other testing.”

Testing to UL and IEC standards “used to cost $120,000 to $130,000,” Rai said. “Now it is more like $70,000 to $80,000.”

The price drop, he explained, is because there are fewer new products and “a significant capacity for doing the tests, based on the demand of two years ago.” Smaller labs, he said, “are exiting the business. Larger labs are consolidating. Change is as considerable on the testing side as on the manufacturing side.”

The cost for bankability testing is “similar to the safety standard test,” Rai said, though it could be as low as $10,000 or $15,000 if we are doing a very basic test or trying to test a very specific type of function.”

This is the second article in a series on module testing procedures. See part one here.