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Post Info TOPIC: Solar wattage.
brian


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Solar wattage.
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rockylizard wrote:

Gday...

I have two 80watt panels on the roof - got two for the reasons already stated ... dimensions better fitted the area on the roof of the van.

However, I have often heard this comment - "A little bit of shade over just one cell on the panel can drastically reduce its output and therefore overall daily performance"

I have been really pleased with the performance of my two panels. Even when it is fully overcast I get 20+volts from the panels and around 7Amps. 

If they perform so well on overcast days, why would shade on only ONE cell of the whole panel be so detrimental?

I ask the question coz I am dumb hmm

Cheers - John

 Overcast is very different to partial shading where one or two cells in the string are shaded whilst the others are in sun. In fact you can often get better performance on a cloudy day than on a sunny one. Reasons are that the light can be very much brighter, and the panel cooler, a solar panel works on light not heat.

A solar panel consists of a number of cells in series which are then in parallel with similar series strings.

So, what happens when you shade one cell in a string is that a number of cells are in series to get the voltage required, and if one is shaded, it can't pass current due to lack of excitation of the substrate, then the current through the entire string (which in a series circuit is the same for the whole string) gets close to the short circuit current of the shaded cell, the string current is reduced to that which can now go through that shaded cell. The other good cells in parallel strings forward biases the unshaded cells in the first string, but the shaded cell becomes reverse biased. When this reverse bias occurs, almost all of the energy created by all of the other unshaded cells is lost through the shaded cell.

Depending on how much reverse bias is applied, the cell becomes over-heated and can actually crack the glass of the panel.

Not a good result.

Be aware that a solar panel can show 20+ volts but still not deliver much current due to lack of sufficient light to cause the electrons in the substrate to move freely.

 



-- Edited by brian on Monday 11th of November 2013 08:19:45 PM

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GaryKelly


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I've read a couple of posts on the forum recently about peeps installing a pair (or even two pair) of solar panels, the total wattage of which could have been reached with just one or two panels. For instance. why would anyone install 2 x 100 watt panels rather than 1 x 200 watt?



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Ford Courier with Freeway slide-on called "PJ". www.aussieodyssey.com
brian


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Could be a few reasons. 

1. Real estate is probably the main one, ie available space on the roof, and the dimensions of that space.

A 100 watt panel can be around 200 mm narrower than a 200 watt one. If you are fitting them along the sides of the RV, then keeping them to the outer edges to avoid shadowing from air-conditioner, hatches, etc, is much easier with the narrower panel. A little bit of shade over just one cell on the panel can drastically reduce its output and therefore overall daily performance. If you take the average width of an RV at 2400mm, then fit a 600mm wide vent or air-conditioner in the centre, you have just 900mm either side for panels, and a 200w panel is very close to and often wider than that, so avoiding shading other than at or close to noon is very difficult.

2. Availability and cost of the particular panels at the time.

3. Weight distribution, a 200w panel will be close to 70% heavier than a 100w one, so spreading them out may be beneficial in some cases.

4. Panel failure - although unlikely - if you damage your single 200w panel you may have no input, damage one of the 100s and you still have half power.

 



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GaryKelly


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Thanks for a quick and comprehensive answer, Brian. You're a gem.

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rockylizard


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Gday...

I have two 80watt panels on the roof - got two for the reasons already stated ... dimensions better fitted the area on the roof of the van.

However, I have often heard this comment - "A little bit of shade over just one cell on the panel can drastically reduce its output and therefore overall daily performance"

I have been really pleased with the performance of my two panels. Even when it is fully overcast I get 20+volts from the panels and around 7Amps. 

If they perform so well on overcast days, why would shade on only ONE cell of the whole panel be so detrimental?

I ask the question coz I am dumb hmm

Cheers - John



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2006 Discovery 3 TDV6 SE Auto - 2008 23ft Golden Eagle Hunter
Some people feel the rain - the others just get wet - Bob Dylan
rockylizard


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Gday...

Thanks Brian ... I think I absorbed all that - but I understand the gist of it

My controller shows the volts on panel and Amps being generated ... so I can "see" how it is going.

I first noticed the "working even when overcast" when I got up early one morning, saw overcast sky, checked to see how the battery was from the night before and noticed the controller was showing 'good work' ... and battery was already showing as at 13.8v.

I have been very impressed with the solar set-up - never need to worry anymore ... almost two years now and not used generator.

Cheers - John



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Some people feel the rain - the others just get wet - Bob Dylan
mr glassies


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just had 800 wt fitted hehehehehehe

dibs


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ken thomas


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thats going more than one better
Ken

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