Power (or not power), that is the question

The power output of a photovoltaic panel is well defined, but customers often have a hard time understanding its meaning and measuring its value. When a solar panel is labeled with a certain power, let's say 100 W, the proper label should be 100 Wp, namely Watt Peak, or the maximum power that can be obtained in standard condition (so called STC), namely at 25 °C of temperature, with a sunlight power of 1 kW/sqm, and with sun rays hitting the panel surface orthogonally. All those conditions do not come together all that frequently, often the cells are much warmer than 25 °C, sunrays are not perpendicular and even more important, eyes are not a good instrument to say how strong the sun is. A full sun as perceived by the human eye can be quite less strong than the standard 1 kW/sqm sun. Obviously there are method and tools to measure the power in the field, but they are not usually available to most customers.

Taking advantage on that, sometimes solar panels resellers or manufactures are tempted to exaggerate the peak power (being quite sure that it will be very difficult for a customer to realize that). This is not only a way to look better than competitors, but a real fraud because solar panels are priced in Watt, therefore increasing the power means selling at a higher price, like a shopkeeper who manipulated his scale.

Actually, having some knowledge in photovoltaic, it is not so difficult to understand if a panel is properly labeled or not, or better, it is not so difficult to pick up the really fake ones. A solar panel is made of a certain number of cells, and the total power is theoretically the sum of the power of the single cells, and in reality a bit less due to the unavoidable power losses that result fromjoining cells together and protecting them with glass or polymers.

There are different kind of cells on the market, and each kind has a more or less broad range of power per cell. As an example, if we consider mono-crystalline silicon 5 inches (125 mm) cells, we can typically have 2.7-2.9 W per cell. Thus, if you see a panel made by 32 five inches mono-crystalline cells sold as a 100 W panel (and you can find it on the internet), well, that gives reasonable ground for suspicion (32 x 2.9 is almost 93 W, and actually even less if we consider the losses).

There are also 5 inches cells that are actually much more powerful, the SunPower cells. They are made with a different technology and are very easy to recognize because of their full black appearance. With SunPower it is easy to reach 3.4 W per cell and thus more than 100 W with 32 cells, even considering the losses. What you can find on the internet is NOT SunPower, but just a solar panel with a fake label.

In Solbian we have a Class A Sun Simulator, namely a tool to measure panels in standard condition, with a lamp that is able to provide illumination approximating natural sunlight. Class A means that the tool has a very small measurement uncertainty (less than 1%) and moreover every month the simulator is checked using sample panels certified and authorized by a certification lab (the so called Golden Samples).

Measuring every single panel is important not only to prevent potential fraud to the customer, but also because the full electric characteristics of the panels can show some manufacturing imperfections, in addition to a wrong power. Thus measuring panels with the right tool is a strong quality control.

Having that machine, you can bet that we are curious to measure products of our competitors, especially when they look too good! Thus, when we found the advertisement of a product quite similar to our SXp96 that was being sold as a 115 W panel, we rushed to buy it. We wanted to understand how it was possible and where we were wrong! To be precise, the panels were not really the same, Solbianflex SXp96 being made with twenty-four 6 inches (156 mm) poly-crystalline cells while the XYZ115 panel with twenty-four 6 inches mono-crystalline cells. Mono is more efficient than poly, thus we were expecting under any circumstances a higher power than our 96 W.

Higher yes, but not that high. To obtain 115 W by using 24 cells means almost 4.8 W per cell. So powerful cells are available on the market, but still the unavoidable losses should have made impossible, at least for Solbian, to reach such a high power. So, eager to discover some technical improvements used by our competitor, we measured the XYZ115 panel in our sun simulator, comparing it with a "humble" Solbianflex SXp96. The results were unexpected even for us, we were looking for a little lie in that 115 W, and instead...

Flash reports for the XYZ115 and for a Solbianflex SXP96

The so called 115 W panel (on the left) had quite the same power of our 96 W panel, actually even a bit less! Was the XYZ115 damaged? We performed also an electroluminescence test on it, and we did not find anything wrong. Only the power was much less than declared, and actually quite close to what we would have expected by using standard power 6 inches cells. Really the same we would have obtained in our CP series, if using 24 mono-crystalline cells.

Is a 20% increase to the real power a fraud? Or only the signal that the XYZ manufacturer is not able to measure the real power, not being equipped with a Class A Sun Simulator? Actually this last hypothesis should worry a customer even more, because the lack of a real test implies it is not possible to check the panel quality, which means a defective panel could reach the shelf, posing a greater risk than the one represented by paying for more power than the real one.

Conclusions? Never trust too exciting declaration. Solar technology is approaching a mature stage and improvements are made step by step, panels that look similar cannot differ too much in power, while they can surely differ in quality.