RV Solar Education > Standard Test Conditions (STC) vs. Normal Operating Cell temperature (NOCT)

Standard Test Conditions (STC) vs. Normal Operating Cell temperature (NOCT)

Standard Test Conditions (STC) vs. Normal Operating Cell temperature (NOCT) Let's say you just purchased a 100 watt solar panel ... how much power (watts) can you expect to get from that panel given the conditions of an "average" day? First, you need to understand how that panel was given that 100 watt rating to begin with.

STC ... Solar Panel manufacturers use what is called Standard Test Conditions (STC). This means they put the solar panels in a flash tester in their factory that has been calibrated to deliver the equivalent of 1000 watts per square meter of sunlight intensity, hold a cell temperature of 25'C (77'F), and assume an airmass of 1.5. This flash test gives them their STC ratings. Consider these as you would EPA mileage ratings ... how many of you actually get the mileage they post?

NOTE: Airmass is the optical path length through the Earth's atmosphere for light from a celestial source. As it passes through the atmosphere, light is attenuated by scattering and absorption; the more atmosphere through which it passes, the greater the attenuation. Consequently, celestial bodies at the horizon appear less bright than when at the zenith. An airmass of 1 is looking straight up from sea level at the sun when it is directly overhead (which it never is in North America).

If you are anywhere north of the Tropics (23.5 degrees of Latitude), the sun is never directly overhead and so it will never give you an airmass of 1. A number had to be agreed upon so 1.5 has become the one used for STC. It is because solar panel manufacturers operate at varying degrees of latitude and altitude that some sort of standard had to be developed so that all solar panels were tested and rated under the same conditions so you have some basis for comparison.

NOCT ... Do Standard Test Conditions resemble "Real World" conditions? ... Not Hardly! Because of this, when utilities and municipalities are trying to calculate real available wattage on an average day (in order to issue tax credits, etc.) they use what is called Normal Operating Cell Temperature (NOCT) ratings. NOCT recognizes a bit of reality and assumes the following: 800 watts per square meter of Sunlight Irradiance, an average of 20'C (68'F) Air Temperature, an average wind velocity of 1 meter per second (2.24 miles per hour), with the back side of the solar panel open to that breeze (as opposed to being on a roof where heats builds up under the panels).

Given those conditions, what is the Temperature of the Solar CELL (not the air)? The average was found to be about 48'C (118.4'F). Some panels are a little higher and some a little lower. What you need to know is that ALL solar panels experience a voltage drop as they heat up. Since Volts x Amps = Watts, the power of the solar panel diminishes as they heat up.

Now think about this ... since most solar panels in use on RVs are laying flat on the roof, they are operating at even higher cell temperatures than what is expected from NOCT (as described above) with the back of the solar panel open. This is why you want to keep the panels raised up off of the roof by a few inches or so. Even then, I have measured cell temperatures as high as 70'C (158'F) on a day when the air temperature was only 80'F.

So what does all this mean to you? It means that on AVERAGE, you will be getting about 75% to 80% of the power you paid for. So that 100 watt solar panel you just bought will give you about 75 to 80 watts of power on the AVERAGE day. There will be days when you get the full rating of the panels but those days are few and far between. Likewise, there will be days when it is dark and cloudy and you will get only about 10 to 20 watts out of that 100 watt panel.

Here are some links to the sites where the data I used came from. Spend some time studying these and you will see that panels laying flat on the roofs of RVs do not operate anywhere near their STC ratings. Here's one in my backyard at the University of Oregon (http://solardata.uoregon.edu/SelectCumulativeSummary.html) and here's one at the National Renewable Energy Lab in Arizona (http://rredc.nrel.gov/solar/old_data/nsrdb/). These go back for decades and can be relied upon for good averages.
 
Now let's look at the "Real World Conditions" RV Solar Systems have to operate in, which is the subject of our next topic.