What do solar panel ratings indicate?

Let's imagine you are considering purchasing a 100 watt solar panel and you want to know how much power (watts) you can expect to get from that panel given the conditions of an "average" day.  First, you will need to understand why that panel was given the rating of 100 watts.

Panels are rated in Watts of output. This wattage rating is derived by multiplying a panel’s peak power voltage by its peak power amperage (Pmax = Vmpp x Impp or Watts = Volts x Amps).

STC - Solar Panel manufacturers use what are called STC (Standard Test Conditions) when they evaluate solar panels in a solar simulator called a flash tester.  During flash testing, solar panels are exposed to artificial sunlight with an intensity of 1000 watts per square meter.  (Bear in mind, 1000 watts per square meter of sunlight would only be reached around solar noon, with the panel squarely facing the sun, just after a rain shower has washed all the dust out of the air.) The temperature is 25°C (77 °F) and the atmospheric density is 1.5.  The output of the panel under these Standard Test Conditions is what gives the panel it’s rating.  All solar panels are rated using this same method, meaning that a 100 watt solar panel from one manufacturer will produce that same 100 watts under STC as a 100 watt solar panel from another manufacturer.  Think of the STC rating of a solar panel as similar to an EPA mileage rating for a vehicle.  How often does your mileage exactly match the stated EPA rating?  ...About as often as your solar panel will produce at its STC rating.

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

XJCM SUN SIMULATOR used to verify panel output

If you are anywhere north of the Tropics (23.5 degrees of latitude), the sun is never going to be directly overhead and you will never see an atmospheric density of 1.  Since solar panel manufacturers operate at varying degrees of latitude and elevation, some sort of standard had to be developed so all solar panels could be tested and rated under identical conditions and customers could have a reliable basis for comparison.  A number had to be agreed upon, as a result, 1.5 was adopted for use in the STC. This is roughly equivalent to 500’ above sea level where the majority of the Earth’s population lives.

NOCT – Because Standard Test Conditions hardly resemble "Real World" conditions, utilities and municipalities have adopted what they call NOCT (Normal Operating Cell Temperature) ratings in order to more accurately issue rebates and tax credits.  NOCT incorporates more reality into their conditions by assuming the following: 800 watts per square meter of sunlight irradiance, an average of 20°C (68°F) air temperature and 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 heat builds up under the panels).

When you consider that solar cells are dark blue to almost black, they soak up sunshine and get quite hot.  Under these conditions, average cell temperature (not air temperature) was found to be about 48°C (118.4°F).  Some panels operate a little warmer and some are a little cooler. What you need to know is that ALL solar panels experience some degree of voltage drop when exposed to heat.  Since Volts x Amps = Watts, the power output of the solar panel is reduced when the cells get hotter.

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

The point of this is not to imply that panel manufacturers are purposely trying to deceive you. It is because "real world" operating conditions are so variable that they had to come up with some standard test conditions so that all panel ratings are derived after being subjected to the same conditions as every other panel.  On average, you will be getting about 75% to 80% of the power you pay for. Know that the 100 watt solar panel you are about to buy will only give you about 75 to 80 watts of power during peak sun hours on an average day. There will be days when you get the full rating of the panels but those days will be 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 useful links showing the results of real world solar panel testing.  When you spend some time studying this data you will see that panels laying flat on the roofs of RVs do not operate anywhere near their STC ratings.

University of Oregon (in our backyard)
http://solardata.uoregon.edu/SelectCumulativeSummary.html

National Renewable Energy Lab in Arizona
http://rredc.nrel.gov/solar/old_data/nsrdb/

The data on these sites goes back decades and can be relied upon for good averages.

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