....I pray your indulgence

System · February 2009

Howdy All

O.K., a 'beginners' question here, but as someone wiser once told me, "the only dumb question is, the question you don't ask!"

The question is about solar panels and their rated watts. You see different manufacturers with different wattage ratings on their panels...100W, 120W, etc.

Isn't the total wattage rating on any particular panel a direct representation of the total square inch of panel surface? Or am I oversimplifying?

Thx in advance for taking the time to answer this 'noob' query

Semper Fi!

USMC_Buckaroo

n3qik · February 2009

Re: ....I pray your indulgence

Loosely, Yes. The larger the watts the larger the panel. Now the type of cells the panel is made from will also have a bearing on size also.

BB. · February 2009

Re: ....I pray your indulgence

You are sort of correct... However, you also have to account for material type and construction practices too...

1. Amorphous and Thin Film panels--generally the least efficient per sq.unit. Frequently found in lower cost, shorter life applications.
2. Poly Crystalline Silicon--quite a bit more efficient per sq.unit.
3. Mono Crystalline Silicon--slightly more efficient per sq.uni vs Poly.
4. Gallium Arsenide--Stuff used in space much more efficient than any above.

Then you have construction practices:

a. Round and 1/2 round cells... Silicon wafers are expensive. Round shapes have less waste. However, the "packing" density means lots of a "rectangular" solar panel has no active silicon--causing panel density/overall efficiency per sq.unit to fall.
b. Rectangular cells have more waste, but higher packing density.
c. Evergreen "ribbon" grown cells result in rectangular cells with low waste (no "round corners to cut off", no "saw cuts to cut a "billet" of silicon into thin cells.

Also, you have different ways of collecting the electricity...

I. Fat copper traces on face--lowers resistance to current flow (less I^2*R losses). However, more light that does not reach the cell--lower efficiency of light conversion.
II. "Transparent" conductors/novel connection schemes. Sunpower puts their "conductors" on the rear of the cell--higher efficiency, but "polarization" of the silicon structure causes loss of output voltage and current... Panels must be "Positive" grounded to bias against earth's electric field to discharge(?) the polarization. Nice solution for the panel company--can create issues for Charge Controller and Inverter Mfg. and installers (Negative Ground is the normal assumption for US and much of the world now for DC circuits).
III. Mixing technologies to improve panel electrical conversion. Ultra thin amorphous + mono crystalline panel in one package. Sanyo HIT is an example of this... 4-5 years or so ago, Sanyo introduced this product and the heals of recalls of solar panels constructed with "arsenide". Sanyo's HIT panels are a sandwitch of Silicon and Asi--and at the time, some folks where really concerned what the long term reliability would be... (I have not heard anything + or - about the Sanyo HIT product--but I am not in the solar industry either).

There are also other things that affect solar panel applications... Each solar cell is about 0.5 volts--and you have to put a lot of cells in series to get high voltage, and many cells in parallel to get high current. So, depending on the intended application and the type of "controller" the solar panel is connected too--there is a combination of voltage, current and panel string configuration that will result in an "optimum" solar panel to "load" configuration.

There can also be internal diodes in the solar panels (blocking, bypass diodes). And safety certification (UL/CSA/NRTL) that are many times required by local building codes and insurance companies.

Hope this helps--please feel free to ask more questions. If there is anything of interest, most of the details can be found with a quick web search--or we can probably post some useful links for any specific questions you may have.

-Bill

icarus · February 2009

Re: ....I pray your indulgence

Welcome,

An your right,, the only stupid questions are the ones not asked.

You will find,,on this site some very knowledgeable folks,, perhaps the most so is Bill. His answer is correct. (His answers are (almost) always correct, and always filled with the technical reasons why). I on the other hand (sometimes) have the right answer,, but seldom have the technical reasons and reasonings that Bill has.

My short answer to you question would be,, in short,, bigger panel, bigger wattage output. Remember while wattage is a good rough measure of output,, you have to compare total specs of on panel to another. For example some panels put out rated power at different voltages than others. (Doesn't make one "better" than another,, just different). Also, temperature affects the out put of panels as well,, but panels are usually measured under standard test conditions (STC) so that you can compare apples to apples.

Tony

lamplight · February 2009

Re: ....I pray your indulgence

also, typically, generally, STC ratings are usually 20% roughly more than you will actually get out of them. (im speaking of sharp mono crystaline panels i have)

niel · February 2009

Re: ....I pray your indulgence

that would most likely be about 10%, but it does vary per pv make and model.

Bob McGovern · March 2009

Re: ....I pray your indulgence

So where are manufacturers finding performance gains? Is it in 'packing' transistors, packing the panel itself, better conversion, fewer impurities? Cuz Shell is selling 175W panels that are exactly the same dimensions as the Shell/Siemens 135W panels I bought five years ago; and those were considered pretty hot at the time, actually just culled from their 120W pile if they flash-tested above spec.

Where dat spare 30% coming from?

niel · March 2009

Re: ....I pray your indulgence

bobmcg,
in general, yes to all, and i don't know what " 'packing' transistors" is or means especially when talking of pvs. for shell specifically, you would need to ask them why both pvs with the same dimensions would be so different. big efforts yield small improvements and i'm sure they are constantly experimenting with various things to get more from them, usually at a slightly higher cost to the consumer per watt when they find an improvement and put it into production.

....I pray your indulgence

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