A COMPARISON OF SOLAR PANELS EFFICIENCY


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A DEFINITION AND CALCULATION OF PV EFFICIENCY



The rating table below compares published efficiency of top solar panel manufacturers. I have to note however that the data listed in the specification sheets are normally measured at an optimum load of the module under some ideal conditions called Standard Test Conditions (STC). This is of course convenient to compare apples to apples. However it is not very helpful for the consumers because in practice your system may rarely display its claimed characteristics. If you are interested in some technical details and facts, read on. If you just want to find out which panel claims the highest efficiency, go to the chart.

By definition, operating efficiency η of a solar panel is the ratio between electric power delivered to the load and incident light intensity. It is affected by the four main factors: cells temperature, sunlight intensity, sun angle, and load. In other words, η depends not only on the properties of PV cells as one might think, but also on the environment and the loading conditions. Since this may be confusing, perhaps a little background information is necessary on the operation of the PV devices. Under illumination PV cells generate certain voltage "V". The maximum value of this voltage (Voc) is achieved under open circuit condition. But when a circuit is open no current is flowing to the load. Since power equals to voltage times amperage P=V×I, at open circuit no power is delivered from a panel, and hence its η=0. When you connect a PV module to an electric load, current "I" starts flowing and solar power is delivered to the load. As amps increase, the panel's voltage begins to drop. However, initially, when you draw more current, P and η are still increasing. If you keep increasing the load, at some point power peaks and then start dropping (see facts about solar cell characteristics). Eventually, "V" drops to zero and "I" can no longer increase. Under such condition called short circuit, the module provides the highest current Isc, but its η=0 again since P=0. The operating point at which P reaches a maximum is somewhere between open and short circuit. Typically it is around 0.9×Isc. This operating point is called maximum power point (MPP). At MPP the panel reaches its maximum efficiency (which is what's listed in data sheet and ads). In order to experience the advertised η, you need to draw the current equal to the module's stated "maximum power current" Imp. If your load consumes less or more than Imp, the η will be lower.

Likewise, your PV array will not necessarily operate at STC. For reference, STC is 1,000 watts/sq.m irradiance, 1.5 air mass and a 25 oC module temperature. The temperature greatly affects a PV cell operation as well. Given the fact a typical temperature coefficient of the crystalline silicon cells is around -0.5%/oC, the cells may be loosing 5% power for every 10oC heat up. In direct sunshine, a module may have temperature rise 15-25 oC above ambient. If for example its rated efficiency η=19% at STC, with 25oC overheat it would be only η=16.6%. The bottom line is, the data in the efficiency table are useful mainly for comparison purposes, but the estimation of actual power output is quite complex.

RATINGS COMPARISON TABLE



This ratings chart lists solar panels rated above 100 watt with the highest efficiency at the time of this review (2014).

Manufacturer Model Watts Efficiency (%) Vmp (V) Imp (A)
SunPower X21-345 345 21.5 57.3 6.0
Crown Renewable Energy CR100 100 18.3 17.6 5.7
Xinhonglian Solar Energy XHH135-24 135 18.01 36 3.8
Zhejiang Shuqimeng Energy Tech. Co., Ltd. SE230M-33A / D 230 17.94 46.7 4.9
Sanyo Electric HIP-205BA19 205 17.7 56.7 3.6
Shanghai Pubsolar GYS-280D 280 17.69 36.4 7.7
Sanyo Electric HIP-205BA20 205 17.66 56.7 3.6
Titan Energy Systems TITAN 32-300 300 17.24 53.8 5.5
Enfoton Solar Ltd 60E6M+245F(L) 235 17.06 30.1 8.1
Shanghai Pubsolar GYS-270D (270) 270 17.05 35.7 7.6
Symphony Energy SE-S160 160 16.95 30.2 5.4
Conergy AG STM 210 FWS 210 16.88 40 5.3
Millennium Electric T.O.U. Ltd. MIL-PVT-215W-M03 215 16.84 38.2 5.6
Maaß Regenerative Energien GmbH BN 180-SP 180 16.53 23.5 7.7
Solara AG SM 400SP 100 16.52 17.8 5.6
Centrosolar America SM 400SP 100 16.52 17.8 5.6

Note that we included above only one best model from each listed manufacturer. Some manufacturers such as SunPower, carry many high-efficient models. Our data are provided AS IS without guarantee of any type, neither explicit or implicit. See complete disclaimer linked below. Please contact me if you would like to submit or correct your device information.

Best solar cell efficiency chart We see from the above table that the most efficient model has η≈22%. Note that a theoretical limit of a single-junction cell efficiency at "standard sun" conditions (known as Shockley-Queisser limit) is about 30%. You probably heard of record cell's efficiencies of up to 44%. The fact is, these results were obtained with multi-junction cells under concentrated light, which exceeded thousand times the normal amount of sunlight. For now, the concentrated solar energy technologies are suitable primarily for power plants rather than for home installations. For a reference, the chart to right shows the best PV cell efficiency achieved in research.



REFERENCES AND ADDITIONAL FACTS


Solar Panels: the database of models and manufacturers (note some of their numbers can't be confirmed).
Best conversion efficiencies for various PV technologies.