Hot Solar Panels - Performance Impact

by Jeremy Quirk, Owner - SpotOn Solar - June 2016

HOW HEAT IMPACTS PERFORMANCE.  In this section I will explain and try to help clarify how solar panel temperature (the surface temperature, not the air temperature) impacts performance and power output.  

For instance, know that when the surface temperature of the solar panel is above the standard testing condition of 77F a solar panel will experience a linear drop in Voltage, and an increase in amperage.  The drop in Voltage which is a larger number, impacts the equation of Voltage x Amerage in a greater manner, and thus typically because of excessive heat on the surface of the solar panel you will have a drop in total power output (Watts that is) at that time.  This will impact charging performance.  

While that may seem bad, the reverse result can be said if the panel surface temperature is below 77F.  In theory, the Voltage of the panel will be slightly higher than the rated standard testing condition (STC) Voltage reading.  You may experience a better charging performance.  

​But in the summer months, or those regions of the Country that are South, in excessive heat conditions, at 12 noon, say it is 89F outside.  If you took a temperature reading with an infrared temperature gun you can expect your solar panel surface temperature to be really hot, likely in the 125-165 F range.  This is well above the STC conditions.    Performance will be less than it would be if the outside temperature was 40F under the exact same light conditions with the sun in the same position in the horizon relative to your panel's position on earth.  

​As an example, on Monday, June 6, 2016 in Cleveland, Tennessee I measured the surface panel temperature of a Solar Power Lid using my Etekcity infrared temperature gun.  The surface temp of the panel read 141F on the meter.  Obviously this is well-above the STC temp of 77F.  Voltage dropped as a result of this excessive heat, and my charging speed was reduced, a slower rate for sure.  

GOOD NEWS still - Don't confuse non-performance versus ideal temperature conditions.  The SPL still charges in high heat but speed will not be as good as it would be if the surface of the solar panel was much cooler.

Coefficiency Factor.  Ask for it.  This is a data number which the panel manufacturer records in its testing to determine how the solar panel reacts to increases in surface temperature.  They need to know the impact to the performance of the solar panel.  Normally you can ask for specific data that takes into consideration heat or cold.  This is called the "coefficiency" factor, or sometimes coefficient factor, number.  

This "coefficiency" data factor that indicates a rate of expected reduced power for high surface temperature, is usually in the .4 - .6% range.  Some technology has panels that are good in high heat, but most mono or poly cells will be in this .4 - .6% range.

All this number means is that for every 1 degree of temp above the standard testing condition of 77F (the solar panel surface temperature), you can expect (light conditions being controlled) the solar panel to perform .4-.6% less in total output of power (Watts) in comparison to the Watt number stamped on the back of the panel.  In other words, your Watts will be less than the ideal, theoretical Watts that were achieved in the ideal STC testing.

A simple example, again light being assumed as controlled for both examples.

a.  The solar panel for our example is rated at 100 watts.  It is 90F outside, and the surface temperature of the panel is measured at 135F.  The co-efficiency factor when asked of the manufacturer, for this panel, is -.5%.

b.  We start with the following equation:

135F surface temp minus 77F (STC surface temp) = 58F

c.  This means that the surface temp of the solar panel is 58F above the standard test conditions ("STC") of 77F.

d.  Keeping light constant, we know the panel loses power as the surface temp is above 77F.  We know the rate is .5% for every 1F above 77F.

In this instance it is 58F above the STC.

58 x .5% = 29

e.  Because we are not in the lab, and it was much hotter than the STC, we can expect, at this point in the real world testing, that the panel will not be able to produce 100 Watts, rather 29 Watts less than the rated 100 Watts.  

Not all solar panel cells perform the same in high heat conditions.  Some perform better in high heat, and some perform better, overall, in a broad range of temperatures. Monocrystalline and polycrystalline panels are more prone to be impacted by high heat than amorphous or cadmium.  The problem is that amorphous starts off with a much, much less efficiency data point (that is the same area of an amorphous or cadmium surface will produce less electricity than a mono crystalline or poly crystalline panel).  So we still prefer and like mono or poly panels for our application.  In cooler temperatures, nothing yet is as good of a performer as a quality mono or poly panel. 

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Helpful Information

"Standard Test Conditions" - What is that?

by Jeremy Quirk, Owner - SpotOn Solar

June 2016

The importance of understanding the lab testing, the standard test condition ("STC") for solar panel data ratings is important and especially so for those curious types that always want to know the truth about how solar photovoltaic panels are tested and rated.  

In my own opinion, STC is not an easy data table for most to comprehend nor that reliable for the application unless you know what they really are.  You have to know how your specific weather and environment impacts performance.  If anything, if all things are constant, then I suppose STC is a good benchmark off of which to judge all solar panels.  But real world performance, I would not rely much on the STC as much as your own experience and testing in your application.  

You can read, and read, and read again, and more, articles found on the internet on this subject, STC for solar panels.  It is somewhat detailed, and some may say a bit controversial as it misleads many.  

For simplification for this site, the things I want to stress you know are the following.  You can look into this further using Google and contacting solar installation professionals in your area.

1.  "Standard Test Conditions" consist of, among other things, overall environment, temperature and light conditions by which the solar photovoltaic panels are exposed and tested in a lab setting in order to provide the public a data reference as to these technical results and performance of the solar panel productivity under controlled circumstances.

2.  The reality is that the ratings for a panel are somewhat misleading unless you understand more about the STC, the control testing usually performed in a lab setting, and the quality of the test and experience of the technicians performing the STC testing.

3.  Most solar panels go through a STC process.  You should insist from the makers of your panels STC lab results and how they conduct them, and whether each panel goes through the testing or sample batches of production.

4.  This is a really simplified statement but for these purposes, enough.  STC conditions for most solar panel testing are at the the following base levels:

a.  The temperature of the surface of the panel, not of the ambient air temp, is set so that it is controlled to be stable at 77F / 25C.

b.  A special light flashing machine, kind of like a laser gun that shoots out bursts of light, is able to simulate the sun's ability to provide the solar panel a "shot" of light at a perfect 1000 Watts / m2 (square meter that is).

c.  Then simultaneously, while the panel is at 77F, the lab technician shoots the light at the solar cells on the panel.  The panel has lead electrical cords that are able to record and measure the results, such as voltage, amperage, and then the calculation of power, called Watts.  Watts is simply a calculation of Volts x Amps.

d.  The test is run several times.  The data is recorded each time to confirm results.

​e.  This includes Voltage ("open circuit" - VoC is what you see sometimes), Amperage, and Watts.

5.  The data collected during the STC are theoretical numbers that the panel could achieve if all conditions were exactly the same as the lab STC.  I stress the data shown on the panel ratings are really more theoretical than real world conditions.

6.  Realistically you will not experience these same data results of your panel unless all the real world, outside environment conditions are perfect, matching the lab conditions of the STC, right at the time of your testing.

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