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~ Updated October 21, 2016

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Solar Panel Efficiency

In order to determine which is the
best solar panel system, one of the most important considerations is the efficiency of the system. Simply put, the efficiency rating of a panel system approximates the percentage of sunlight that hits a solar panel that is converted into electricity. Or, an efficiency rating could be applied to the percentage of the sun's energy that is converted into usable heat, but most often the term "efficiency" is applied to conversion to electricity. One of the main applications of the efficiency rating is for space considerations. Naturally, the most efficient solar panels will take up less space.

There are many factors which can affect the efficiency of
solar roof panels, and these include the age of the panels, the temperature, electrical wiring resistance, the tilt angle of the panels, and spectral distribution. Most of these are self evident, however, spectral distribution refers to the warping effect our atmosphere has on sunlight. For precise measurements it must be taken into account that "pure" sunlight does not actually reach earth (in comparative terms). It is first "filtered" by the atmosphere to varying degrees based on what the solar panel is made of. Credit should be given to monocrystalline solar panels which have higher efficiency due to this filtering.


While you are evaluating different solar panel efficiency ratings it is absolutely essential that you compare the same type of rating. The two most widely used rating systems are "STC" and "PTC". "STC" stands for Standard Test Conditions and is the rating system established by solar manufacturers. They measure solar panel efficiency under uniform sunlight, tilt angles of the panels, air mass and temperature conditions. Here, "air mass" just refers to the filtering effect of the atmosphere described above. The technical specs for the STC rating are 1,000 Watts per square meter solar irradiance, 25 degrees C cell temperature, air mass equal to 1.5, and ASTM G173-03 standard spectrum. The actual rating number represents the average number of Kilowatts of output per day.

In contrast, the "PTC" rating refers to PVUSA Test Conditions. These resulted from the Photovoltaics for Utility Scale Applications project, which was begun in 1986 and is a joint venture between Pacific Gas & Electric, the Department of Energy and other utilities across the USA. The technical specs for the PTC rating are 1,000 Watts per square meter solar irradiance, 20 degrees C air temperature, and wind speed of 1 meter per second at 10 meters above ground level. The PTC rating number also represents the average number of Kilowatts of output per day, and without getting into a huge analysis, just know that it is generally believed that the PTC is a better and more precise rating system. This is because the PTC testing conditions are considered to be more reflective of "real world" solar and climatic conditions. In addition, the PTC rating also takes into account the inverter. Consequently, the PTC number will be lower than the STC number. While PTC is preferable, always make sure that whatever numbers you have available for comparison are based on the same rating system. Continued below....

Solar Panel Efficiency


Counter-intuitively, an increase in air temperature actually decreases a solar panel's efficiency. The reason for this is somewhat complex, but we begin by noting that as the temperature increases, the conductivity of the solar cell material also increases. This, in turn, will cause the electrical charge within the solar cell material to balance out. When the charge balances, that makes charge separation more difficult. And when charge separation is made more difficult, the voltage across the cell goes down. Voltage is akin to water pressure in a hose, so if that goes down then less water comes out, or here, less electricity is produced.

Now, to be fair it should be noted that an increase in temperature does cause an increase in the mobility of electrons, which causes the amperage (flow of current of electricity) to increase. That means more electricity, but this increase is much more than offset by the aforementioned decrease in voltage.

In fact, it has been estimated that solar cell efficiency goes down by about 0.5% (in crystalline cells) for every degree centigrade the temperature rises above 25 C (77 degrees F.). So, as you can see those who live in climates that frequently reach temperatures above 25 C should be aware of this effect.


Fortunately, there have been a great number of
solar panel efficiency improvements over the last several years, and many more are doubtless on the way. A short survey of some of them would have to include mention of so-called "concentrator systems". These systems increase the light intensity by using concentrating optics. Higher intensity light per amount of space increases the efficiency of the cell, especially when they are now-cost-effective GaAs cells.

Another improvement in efficiency has come with the use of multijunction cells. These contain several layers of silicon, with each layer tuned to capture different light frequencies. Extracting energy from multiple light frequencies maximizes the production of energy from each solar unit of light coming from the sun. These are among the most efficient solar panels available. And while these are super efficient, they are also very expensive, so before we see them on a mass scale further developments will have to be made to bring the price down.

One of the latest and most exciting innovations has come in the form of what are called "microinverters". In a typical solar panel system, the inverter is what converts the DC power generated from the sun's energy into AC electricity for use inside the home. Until now, the technology has been to have one inverter for the entire system, and then have cords running from each panel to the inverter for power conversion.

With the microinverter, though, each panel has its own mini-inverter. This will not only greatly reduce the cost of having to run so many cables back to a single inverter, but it will also increase the efficiency. Since each cell will have its own attached microinverter, the electricity will not have to travel back to a single standard inverter. This increases efficiency because the longer electricity has to travel the more power will be lost due to resistance.

Finally, perhaps one of the more unusual efficiency developments in recent times involves the use of "building integrated PV", also known as "building-applied PV". With this approach, instead of attaching solar panels to building structures, the buildings are actually partially made of the solar panel materials themselves. At first, this will more commonly be seen on such things as skylights or facades, in addition to certain types of roofs. It is easy to see the potential for enormous savings in the cost of the regular building materials no longer needed, along with the extra power generation of the solar panels, which will be able to cover a much wider area and generate even more electricity.