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 10^oC 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 25^oC 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 best solar panels rated above 100 watt with the highest efficiency at the time of this review (2017).

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
Maaߟ 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.

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

Best conversion efficiencies for various PV technologies.