The 5 most important things to look for in solar panel data sheet (+infographic)

By Evangelia Paraskevadaki Sunday, 8 December 2013
Whether you want to buy solar panels for your home or you are one of the solar panel installers, this article will guide you effortless through the process of solar panel evaluation, so you can eventually select the best solar panels for your installation.

Nowadays there are too many solar panel manufacturers in the world. Since the solar industry started to grow in Germany, the US and Spain, everyone wanted to participate and solar panel manufacturers multiplied. If you visit the ENF solar panel database you will find that there are 1162 registered manufacturers of crystalline solar panels alone!

And guess what... Only few of them have been around for more than 8 to 10 years, so you can not entirely depend on previous experience. In addition, it is possible that a manufacturing company will not have integrated production, meaning that they only assemble the various components (glass, cell, back sheet etc.) and they buy wafers and other materials from other companies. So, most of the times, all you have is the solar panel info sheet.

The question is: How could you evaluate the quality of a a solar panel based on the manufacturer's data sheet?

Let's see what Detective Solar has to say!


Now we assume that we have beforehand the data sheet of a polycrystalline solar panel.

1# solar panel guarantee

In the front page we find something that says how good is the solar panel and how many years guarantee is provided for workmanship and performance.

Most manufacturers provide 10 years guarantee for workmanship and 25 years guarantee for performance. So don't be impressed. The important thing is to look for the word “linear”. If you find it, then check out the percentage of nominal power output that this linear reduction of power output applies to. Got confused?


solar panel 1 - “We provide a 10 year guarantee for workmanship, a 90% power output guarantee for the first 10 years, and 80% power output guarantee for 25 years of the solar panel 's life.”

solar panel 2 - “We provide a product guarantee of 10 years. We also provide a linear performance guarantee of 90% for the first 10 years and of 80% for a period of 25 years.”

What's the difference? Let's see:

The manufacturer of solar panel 1 guarantees that he will replace the panel in the case of a defect that is due to workmanship (the same as product guarantee). He also guarantees that if you test your solar panel after 10 years of operation (or delivery) you will find that the performance has not been reduced more than 10%. If you measure it after 25 years you will find that the performance has not decreased more than 20%.

This way, it is possible for solar panel 1 to degrade to 90% in the first year and operate for 9 years with 90% power output, hence, produce 10% less energy than the one you signed for. Early degradation is very common. Then at the 10th year, the performance could degrade at 80%.

The manufacturer of solar panel 2 has the same product guarantee but the performance guarantee is linear. This means that the power output of Module 2 will not degrade more than 1% per year, hence, the solar panel will utilize its potential more efficiently and produce more energy for a period of 25 years.

Since the problem of early degradation is common, some manufacturers guarantee performance for the first year. The say for example, that the power output will not degrade more than 3-5% for a period of one year from the date of delivery.

2# solar panel power tolerance

What is power tolerance? The power output presented in the solar panel info sheet, i.e. a module of 260W, is the nominal power of the solar panel. But in a production line, all panels could not be the same meaning that there will be a diversity between the panel 's data sheet ratings and the actual characteristics. So when the panel of 260W rated power is delivered to you, it could be of 265W or 255W. That's a -/+5W power tolerance.

Manufacturers include power tolerance in the solar panel data sheet at the front page or under the table of electrical characteristics. Most solar panels have positive and negative power tolerance of -/+5 or -/+3 Watts but some have only positive tolerance, for example -0/+5W, ensuring stable high energy output.

3# solar panel certification

You should look for IEC 61215 and IEC 61730 certificates. The solar panel data sheet will refer to them and the manufacturers, following your request, could provide the certification documents. You should look that the module is certified for the latest acceptable versions of these standards.

IEC 61215 describes the quality requirements for solar panels and the tests that should be implemented in order to determine the mechanical and electrical characteristics that are presented in the solar panel info sheet and are essential for the product's useful life (irradiation effects, thermal and mechanical stress, abnormal stress is used to simulate accelerated aging)

The IEC 61730 Standard defines the safety requirements and has two parts. The first part determines if the solar panel complies with the general safety aspects such as distance from the conductive parts to the frame and other. Part 2, describes the tests that are specific for the solar panels.

The solar panel should be approved by Class A of the IEC 61730 standard applied for unlimited access, hazardous voltage, systems with hazardous output, direct current voltages higher than or equal to 120 Volt DC and Protection Class II.

4# nominal operating cell temperature

NOCT stands for Nominal Operating Cell Temperature. Nominal electrical values presented in the solar panel data sheet are measured at 25 °C, 1 kW/m2. But when in the field, solar panel will operate at much higher temperatures, so performance should be measured at different conditions from STC (Standard Test Conditions). NOCT is the temperature that the solar panel will reach when open circuited and at conditions of 800 W/m2 irradiance, 20°C temperature, 1 m/s wind velocity and mounted with open back side.

If the NOCT is low (~ 30-40 degrees Celsius) the solar panel is more stable and operates more efficiently. Typical solar panels have a NOCT of 47-48 degrees Celsius and the worst panels a NOCT of 55+.

5# solar panel temperature coefficients

In solar panel data sheet you will find a table that is under the title "Characteristics" or some other similar word. There, the temperature coefficients will be listed, meaning the deviation of the solar panel 's electrical characteristics, voltage, current and power relative to the increase of temperature per dergree Celsius. As you would see, the coefficients of voltage and power are negative. This means that solar panel 's voltage and power output decrease when the temperature increases. The coefficient of current is positive, so the current increases when temperature increases.

The important information is the power decrease. You want to know how much power loss your system will present when cell temperature rises above 25 degrees. And if you consider that a cell temperature of 25 degrees means ambient temperature of 10-15 degrees, you could see that losses due to temperature will be present almost all year long.

So the temperature coefficient should be as low as possible. A typical solar panel has a coefficient for power at about -0.45%. It would be better if you found a panel of -0.4% or even 0.35%.

So, if you want to buy solar panels or become one of the solar panel installers, now you know.

Image courtesy of Salvatore Vuono /

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Eva Paraskevadaki

I'm Eva Paraskevadaki. Freelance researcher and technical consultant. People hire me for grid load flow and impact studies, PV system sizing and energy assessments, electrical designs and layouts, development of innovative power systems and much more. I also enjoy writing about the technical aspects of solar power in my blog.

11 comments to ''The 5 most important things to look for in solar panel data sheet (+infographic)"

  1. Interesting to note this valuable data regarding the proper selection of solar panel. Keep it up, you are educating the entire world,when every country is encouraging its citizens shift attention towards renewable energy sources.

    1. Thank you very much Zimbeva! I am trying to share my views, because progress, I believe, comes through knowledge and creative discussions :)

  2. Very informative and worthy post. Thanks for the sharing such a precious updates with us..installers

    1. Thank you Malik! As I see from your link, you are active in solar power systems for homes and businesses. Could you share your views with us? Are there some panels that are prefered from installers due to specidic qualities?

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  4. Thank you so much for your invaluable insight! I enjoyed reading most (if not all) of your articles.

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