Pacific Islands Construction has found that most consumers are being provided with test data comparing one module to another with very little explanation of what these tests really mean. When confronted with these tests, consumers tend to believe they indicate real-world performance. Many, in fact, will refer to the PTC test data or the STC test data to substantiate their module as being one of the best.

Here’s an explanation of what these two tests are and what they really mean:


PTC refers to PVUSA Test Conditions, which were developed to test and compare PV systems as part of the PVUSA (photovoltaics for Utility Scale Applications) project.

PTC are 1,000 watts per square meter solar irradiance, 20 degrees C air temperature, and wind speed of 1 meter per second at 10 meters above ground level. PV manufacturers use Standard Test Conditions, or STC, to rate their PV products. STC are 1,000 watts per square meter solar irradiance, 25 degrees C cell temperature, air mass equal to 1.5, and ASTM G173-03 standard spectrum. Cell temperature in a controlled 77 degrees F (25 C) will produce more power than 20 degree C air temperature. The PTC rating, which is lower than the STC rating, is generally recognized as a more realistic measure of PV output because the test conditions better reflect “real-world” solar and climatic conditions, compared to the STC rating. All ratings in the list are DC (direct current) watts.

What is important to note is cell voltage drops as the temperature of module increases. Cell voltage drops about 0.08 volts per degree C in environments that exceed 25 degrees C. Using Ohm’s Law, volts times amps is equal to watts, which equals power, so a reduced voltage means reduced watts. During our hot Hawaiian days when our solar modules increase in temperature, the voltage drops, decreasing the power they produce.


Do not be confused and accept either of these two tests as true indicators of actual performance expectations. Neither of the two tests account for all “real-world” losses. A factor such as panel degradation over time cannot even be considered in a test of this nature. A recent study by the Renewable Energy Laboratory suggests that both single and multi-crystalline field-aged modules degrade on average by .7 percent per year, losses caused mainly by UV absorption at or near the silicon surface. Solar systems in the real world degrade and will produce lower outputs for some of the following reasons: soiling, shading, module mismatch, wire losses, inverter and transformer losses, shortfalls in actual name-plate ratings, and high-temperature losses for arrays mounted close to or integrated within a roofline.

Another concern is the test modules themselves and what is being provided to the testing facility. Originally the PTC test was allowed to be tested in the manufacturers’ facilities, but when the tests were performed in independent labs, the scores were found to be lower than those performed by the manufacturer. Manufacturers also have the ability to provide the module they want to have tested, which probably is one that has been tested beforehand that they know will provide better results. Until modules are randomly pulled off the assembly line and a test can be developed that really represents all of the real-world conditions, don’t place too much stock in these tests.

Pacific Islands Construction believes that properly installed and designed PV systems will perform at higher levels. The company also believes modules designed for Hawaii will provide the best rate of return on your investment in the long run.


Call Pacific Islands Construction at 841-7756 for a free consultation regarding your solar, roofing or construction project.


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