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A specialist from one of the companies distributing and retailing solar panels brought up a question for discussion that is not usually very specifically talked about but was raised by a client of theirs. The client was looking for the most efficient panels in terms of peak power (rated watts)  per square foot of the panel (module) the way it was commercially available.

He was not interested to hear about the comparison between the different technologies in particular because they are usually compared on cell level and the data available is usually from scientific lab test  not necessarily directly related to the realistic efficiency of the finished panel. When cells are actually implemented in the configuration of the panel there might be numerous factors additionally affecting the power-to-physical-area efficiency of the panel. Different models of panels are also built on different grade of cells so many time you can have one and the same physical shape, area and cell configuration gaining very different rated power (watt peak). How much space (area) is left  between the cells in the panel can also influence the over all watt per square foot performance.

The different solar photovoltaic cell technologies are usually given a percentage physical efficiency number that represent what part of the light energy is turned in to electrical energy. Monocrystalline cells usually are said to be with physical efficiency of about 16 – 17%. Polycrystalline – 14 – 15%. So, often in discussions it is assumed that monocrystalline panels, too, are more area efficient. As we will see our study shows that this does not need to be the case.

Because polycrystalline cell based panes use square (or rectangular) cells they use better the surface area of the panel for light to electricity conversion.

The monocrystalline cells are cut off from round wafers and though they usually have for edges cut off from them for the purpose of not removing too much of the expensive wafer they are not turned in to square cells but in to squares with round edges. When these cells are put in to a panel there are small areas between the edges of the cells that are left unused.

In thin-film panels usually most of the area of the panel is covered by a photovoltaic surface. The configuration of what are effective cells in them may be more complex but there are better opportunities in thin-film panels to use larger part of the surface of the panel efficiently. With thin-film the actual photovoltaic elements are not as physically efficient though – the commercially widely available ones are only know to have 7 – 9 % physical efficiency. So it will be hard for them to compete with the mono- and poly crystalline ones.

You probably already know that SolarByTheWatt.com in general maintains that the economic efficiency of photovoltaic equipment (like panels) is the most important number because this defines the economic efficiency of a whole project.

Surface area efficiency (watt per sq. ft.) can only be critical, or more critical, when there is a limitation of how much area can be used for placing the panels and there is a requirement for the peak power of the whole project. Often this could be the case with residential projects where we intended certain peak power already and the roof area is simply not enough for us to go with the most cost efficient technology (possibly thin film).

Let’s look at the practical numbers for commercially available panels and compare the surface area efficiency they have. We will get the numbers for peak power rating in watts for the panels and the actual physical surface area calculated from the reported dimensions of the panel.

In the following table we have put the number for panels that available at online retailers. If any of the readers know of a model that might show interesting numbers they are welcome to submit data and links through the comments form or the Contact Us page and form on the website.

We were trying to select panels for which we calculated highest watt per square foot efficiency. That is marked in the table as power density.

This is a key to the different models – how they correspond to a brand name and a website of one possible vendor.

SMD – SolarMaxDirect – http://SolarMaxDirect.com
STP – Suntech Power http://store.solar-electric.com
KAN – Kaneka – http://store.altestore.com
KD – Kyocera – http://store.solar-electric.com
PVUD – Mitsubishi – http://aeesolar.com
SW – Solar Wordl – http://aeesolar.com
NT – Sharp – http://sunwize.com

We reiterate here that we were looking for the data from the specification sheets that come on different product pages of different vendors. Apart from the ordering by power density, or area efficiency –  watt per sq ft, we did not have any other criteria for picking the model or the places they can be purchased. Please do feel free to submit your suggestions for us to include other examples.

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