As introduction, you should read our article on the types of solar panels here.
STC (Standard Test Condtions) and PTC (PVUSA Test Conditions) are the two methods of testing module performance. The STC and PTC ratings of solar panels are one of the ways of comparing the performance of one brand and/or type of solar panel to another. They are very important when more than one solar panel is to be used together in an array.
STC stands for 'Standard Test Conditions'. After a product is manufactured, it is often subjected to a standard test under a fixed set of ideal conditions. In the case of solar panels, the STC is the standard test for solar panels. Every solar panel must pass an STC test before it can be certified by certification agencies and introduced into the market.
Manufacturers use STC testing to ensure that solar panels with similar energy output can be sold and used together.
The STC rating of a solar module is determined by carefully controlling sunlight (and sometimes Ultraviolet rays) and temperature in the testing environment. The light source in the laboratory is calibrated so that precisely 1,000W per square metre of solar light falls on the solar panel. The temperature of the solar cells and the ambient room temperature are both set at 77 degrees.
For instance, a Canadian Solar 180W panel is 180W (STC). An array made from 10 of these panels would be 1,800W (STC). In other words, the STC rating of a solar panel is its output capacity under ideal conditions. It is thus, the name plate value of the solar panel and is given as the wattage of the module.
PTC refers to PVUSA Test Conditions. It was developed by a California-based solar research and development partnership between the US Department of Energy and several major utilities. This group was established in 1986 and was known as Photovoltaics for Utility Scale Applications (PVUSA). They merged with the California Energy Commission in 1997 and ultimately dissolved in 2000. Although the PVUSA no longer exists, their method of rating module performance was found to be more accurate than the STC rating system and is still in use today.
The major difference is that while STC uses a steady temperature of 25° Celsius, PTC uses a temperature of 20° Celsius combined with a calculation to measure module performance.
The PTC rating, is often lower than the STC rating because test conditions reflect a non-ideal and realistic solar and climatic conditions. Therefore, it is generally more recognized as a more realistic measure of the PV output. It is the benchmark for calculating the total output to be expected from the system.
In the case where a PTC value is not specified, a good rule of thumb is that the PTC rating is about 10-15% less than the STC rating.
There are more significant things to consider when acquiring a solar module and there are other factors that will reduce the output and efficiency of a solar system. For instance, shading of the panels, wiring inefficiencies, environmental conditions and module heating all decrease the efficiency of solar panels. Thus while the PTC ratings offer a realistic view of photovoltaic output, they do not indicate an exact energy production.
However, when designing a solar system especially those that require an array of more than one panel, SolarKobo engineers use the difference between STC and PTC to determine the number of modules that will deliver the required amount of energy, in keeping with the user's budget and space.