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TLDR: Please share your experience on how much shading can affect electric current produced by a solar panel.

I got some time to spend with my parents, due to this Corona situation. So, I went to our farm house and setup a solar panel system there. I used 380Wp Mono crystalline PERC panels and the voltage, current I am getting from each one of those panels are satisfactory. Now, I moved back to my place in a city and I live on 13th floor of an old 20 storey apartment. I decided to setup some solar panels for myself too.

I used the same panels that I used in the farm house. The difference is, since its an apartment, I dont have access to roof top. I can put my solar panels outside the window over the ledge, which I did. There is a grill outside the window which casts a shadow. I believe that may be the reason for which I am not getting a decent current output. Can you please take a look at the picture and let me know how much of an impediment is that shadow, from your experience?

This is a 380Wp Mono PERC panel. I have observed it to produce close to 330W under decent sunlight in the farm house where the array is roof top mounted, even though it is a little cloudy at this time of the year. The highest I got from the same panel in my apartment, with those grill shadows is 236W. enter image description here

enter image description here

Update. As per my coordinates, I should be facing 90 degree to south. I was a little off. Besides, the elevation was not right. Look what I am getting once I fixed that.I got a maximum of 379W out of a panel rated for 380W under STC, hovering somewhere around 350-370W. The temperature at the time of taking the reading is 31 celsius. STC is 25 celsius. These seem to be good quality panels.

379W out of a panel rated for 380W under STC. The temperature right now is 31 celsius

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    \$\begingroup\$ Vertical orientation, rather than angled toward sun, may be a another factor. \$\endgroup\$
    – mbedded
    Oct 27, 2020 at 12:43
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    \$\begingroup\$ The percentage blocked by the grill is not that much. You need to consider the angle to the sun. \$\endgroup\$
    – Mattman944
    Oct 27, 2020 at 12:45
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    \$\begingroup\$ What compass direction are they facing? South is usually optimum. \$\endgroup\$
    – Mattman944
    Oct 27, 2020 at 14:00
  • \$\begingroup\$ @Mattman944 It's south facing \$\endgroup\$
    – de_xtr
    Oct 27, 2020 at 16:48
  • \$\begingroup\$ Related video on eevblog: youtube.com/watch?v=AbxHoQF4ADk Result: Shadowed area can be tiny, but the output drop may be significant. \$\endgroup\$
    – asdfex
    Mar 8, 2022 at 10:52

3 Answers 3

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Shading is the biggest factor in power loss, and it is widely dependent on the panel design.

When a cell is shaded, it will significantly hinder the passage of current. The cell will heat and can actually cause damage, and even fire in some cases.

In a solar panel, the cells are arranged in "Strings". In technologies up to around 2020, most panels would consist of 1 string of cells connected in series.

Within the string, usually on the junction box of the panels, bypass diodes are placed. In most designs, you have 2-3 bypass diodes. They are connected to different regions of the string, usually where they meet at the top. These bypass diodes roles are to allow the current to flow, effectively bypassing part of the string when there is shading or other issues like breakage of a cell.

enter image description here

The power loss due to shading is very dependent on the shape of the shading on the panel. In this design, if you shade the last bottom row of the cell, the power output of the panel will be very significant. If you shade a column of cells in one of the sides, the power output will be about 2/3, which is actually higher despite the fact that you are shading more cells.

Some very low-end panels, may not have bypass diodes at all, or a single one. In that case, any shading will have significant effects.

Solar panels are characterized by their I-V characteristics. The I-V curves below show the effect of shading, and the effect of the bypass diode. enter image description here

In 2020-2021 forward, panel design has seen a dramatic change in the industry with what is called "half-cut" cells, where now 2 sets of strings are connected in parallel. This brings some efficiency and production improvements.

enter image description here

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  • \$\begingroup\$ Thanks. Shedding was indeed a problem. Apart from that the azimuth and elevation were also slightly off. Now, with those problems taken care of, I am getting a maximum 379W out of a panel rated for 380W. I had checked the solar radiation at that time and it was a little less than 800W/sq m. I must say, I am happy with the panels I bought. \$\endgroup\$
    – de_xtr
    May 27, 2021 at 5:59
  • \$\begingroup\$ The rated power is the STC (Standard Test Condition) which is at 1'000W/m2 and 25°C. In real conditions, it is different, the power will drop as temperature increases. So you may have big variation of measurement depending on temperature and irradiation intensities. The only "good way" to accurately measure the power output of a solar panel is to use a specialized system such as these: avalon.solar/nexun \$\endgroup\$
    – Damien
    May 27, 2021 at 7:09
  • \$\begingroup\$ Yes, shadow can be a huge problem. Even the shadow of a single cable can drop the output of a whole system by 20%! See in this video by Dave Jones eevblog: youtube.com/watch?v=AbxHoQF4ADk \$\endgroup\$
    – kruemi
    Mar 8, 2022 at 13:52
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Partial shading can cause massive reductions in power. I looked into this a little, because the best place for a solar panel on my little yacht is under the boom, which often partially shades the panel.

So I measured the short circuit current with and without shading:

enter image description here

2.44A unshaded but only 0.56A with shading covering the entire width of at least one cell.

Interestingly, 1.44A with shading covering only part of the width of any single cell. Still quite a significant loss. My impression was that the axis of the shading mattered; that covering the whole width was more serious than covering the whole height (parallel to the connection wires) but that may have been because this panel uses 1/3 height cells and looking back at my results I have none where I only covered half the height of each cell. Careless experiment design!

Still, it might be interesting to see if your results depended on the orientation of the panel with respect to the grill.

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  • \$\begingroup\$ Interesting results indeed. Thank you for sharing. I will test my panel, without the shadow of the grills, and then at specific angles with respect to the grill and share the results. There is a low pressure developing in the region. I will have to wait for a couple of days to get a clear sky. \$\endgroup\$
    – de_xtr
    Oct 27, 2020 at 15:41
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    \$\begingroup\$ This is exactly what I found on our sailing yacht as well. That's the reason why I made my solar panel (somehwhat) movable (of course it's fixed on the cabin roof when sailing). At home I noticed that the small amount of shading from trees I get in the morning during wintertime also has a measurable effect on yield. \$\endgroup\$
    – StarCat
    Oct 27, 2020 at 16:13
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The shading is very probably not the main reason you are seeing a reduction in power; I would be surprised if the reduction was more than, say, 5%. The angle at which the panels sit has more influence than that.

I'm guessing your solar panels are sitting at about a 5° angle, which is probably (depending on where you live) quite far from the optimum angle.

Taking The Netherlands, where I live, as an example: the optimum angle here is about 37° on average; it will change with the seasons (14° in winter, 62° in summer).

Setting up the solar panel at 5° would reduce the power delivered considerably, especially in summer; the panels simply "catch" less sunlight. For example: when you're off by 45°, you only get 70% of the maximum power.

The angle from the geographical south, or north in the southern hemisphere, also plays a role, of course, in a similar way.

There's a calculator to find the optimal angle here; once you found the optical angle you can do the math and find how it compares with the angle at which you set up the solar panels, and set the panels' angle. You can also adjust them by eye, of course. It does require more space, but I can't help you there.

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  • \$\begingroup\$ My coordinates are 19.155469, 72.885435 (Well now you know where I live) As per my coordinates the optimum angle from vertical for the month of September is 71. I had calculated that before setting it up. Thats what the panel is aligned at right now I have not used fixed channels because its not finalized yet. Its 63 from vertical for October, going down to 55 for November. I will try that. But, even in September, I was nowhere near to optimum. \$\endgroup\$
    – de_xtr
    Oct 27, 2020 at 13:26
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    \$\begingroup\$ I have misinterpreted your photo then; I could have sworn the panel was standing up almost vertically. I'll leave the answer for now; it may be useful for others. Given your coordinates, maybe smog is a factor. In Delhi the average level of attenuation due to smog is 12%. Average, mind. \$\endgroup\$
    – ocrdu
    Oct 27, 2020 at 13:39
  • \$\begingroup\$ Photo taken from a mobile phone at a close angle does not do justice to it. It is 71 degree . I think it has got something to do with my panel. If I cover just one block out of the 6*12 blocks, the power drops to half. I will check the diodes. \$\endgroup\$
    – de_xtr
    Oct 27, 2020 at 13:51
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    \$\begingroup\$ Fully blocking one solar cell in a panel from the sun has a large influence; covering a very small part of a cell like shown in the photo, not so much; not what you are seeing. I actually measured that on a balcony with a similar railing, BTW. \$\endgroup\$
    – ocrdu
    Oct 27, 2020 at 13:57

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