Solar panels are tested under different conditions to give them a rating. Which one of these should you be looking at?
Shopping around for any new technology always means that you will be learning new terms related to the product.
Whether you’re talking horsepower with a truck or RAM with a computer, there is always something new to learn.
Solar is no different.
If you are in the market for some new solar panels, it may be a good idea to check into the STC, PTC and NOCT solar panel rating system terminology and statistics.
This way, you’ll have a better understanding of what is involved with the paneling and what is best for your particular needs.
Before diving into the panel ratings, you should know what STC, PTC, and NOCT are.
These systems are designed to rate the effectiveness of the paneling in regards to absorbing energy from sunlight.
STC is short for Standard Test Conditions. This is where the paneling is measured in lab conditions of 1000W per square meter of sunlight.
By using a fixed set of conditions, all solar panels can be more accurately compared and rated against each other.
PTC is is short for Photovoltaic USA ( PVUSA) Test Conditions, and the specific rating system used in California.
This rating was developed to test and compare PV systems as part of the PVUSA (Photovoltaics for Utility Scale Applications) project.
California has some of the strictest energy ratings and requirements in the United States, which is why it has its very own solar panel rating system.
The Sunshine State only makes use of PTC as the benchmark for solar paneling installed within the state.
In “real world” conditions, PV panels generally operate at higher temperatures, and lower insulation conditions. If you think of solar panels, they are all dark, either black or blues.
This causes a lot of heat soak and heat build up underneath the solar panels. Because of this, the average cell temperatures (not air temperature) can be as high as 48°C (118.4°F).
If you look into some data, you will notice that flat lying panels don’t operate anywhere near their STC ratings.
Because of this, a NOCT rating was created, which stands for Normal Operating Cell Temperature.
This rating incorporates more realistic outdoor conditions, and is used to issue rebates and tax credits more accurately.
This test keeps the back of the solar cell open to the breeze, as opposed to panels which lay flat on a roof and build up heat.
The ratings use several different variables in order to provide some more real world conditions rating.
After all, there are elements at play outside that are not present inside of a factory floor.
STC is when solar panels are evaluated in a solar simulator called a flash tester.
During flash testing, solar panels are exposed to artificial sunlight, simulating conditions that would only be reached around solar noon, with the panel squarely facing the sun… just after some heavy rains cleared all the dust from the air.
Here are the three standard test conditions:
Of course, there are many different temperature variables, so the solar paneling may increase or decrease in functionality based on both the air pressure and the temperature outside.
The PVUSA Test Conditions include a few additional variables when measuring the levels of the solar paneling.
While it does still use the 1,000 watts per square meter variables and the 1.5 air mass, it sets its temperature to 20°C ( 68 degree F ) ambient temperature, positioned at 33 feet above ground level, and a wind speed of 1 meter per second.
The Normal Operating Cell Temperature, is classified by the temperature solar panels reach from open circuit or open back cells.
This test differs in that it makes use of wind velocity rather than mass of air.
The following conditions are used to calculate the NOCT rating.
With all the variables to include, what is important to understand is that cell voltage decreases while the temperature increases.
Therefore, the 20 °C temperature usually provides a more true to life average daily temperature, including both day and night.
Essentially, as solar panels heat up, some of the energy is lost due to this increase in temperature (similar to a cell phone burning through its battery storage faster when it is running hot).
This means that a solar panel is able to absorb more solar power and absorb more energy at 68 degrees than at 77 degrees.
All solar panel rating systems are helpful, and when you look over each panel option you can see which one performs better and can generate more energy.
If it is available, however, look for the PTC or NOCT option, simply because it offers a more “true to real world conditions” rating.
Johnny K 
