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Partial shading versus reduced solar panel size

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  • Partial shading versus reduced solar panel size

    Okay, so this came up in conversation with a colleague of mine who's converting a van right now. I got really lost in the math, so I'm hoping you guys can put it to bed for me...

    The debate:

    My friend has two obvious spaces for solar panels on the van. They *JUST* fit two 200W panels. On either side of them are standard Dometic Mini Heki skylights, about 1cm from the edge of the flexible panel.

    As the sun moves across the sky, no doubt these skylights will cast a shadow on the panels (especially if the skylights are open since they essentially become almost-vertical translucent sheets about 40cm x 40cm).

    The question then, is whether it's worth fitting 200W panels, or whether they'd actually get more energy from 160W panels (or perhaps even less) which put about 8cm between each of the skylights and panels due to how harshly the panels respond to partial shading?

    I welcome any thoughts/musings/accusations of wild over-thinking/calls for a roof rack in order to just fit 2000W and have done with it etc .

  • #2
    Shadows crossing a panel usually cut their output power down to about 10% of rated capacity. I'm also assuming he is thinking of mounting them flat on the roof, which when UNshaded are now only about 60% efficient and prone to heat damage.

    Save your friend some time and money - the panels need to be somehow angled upwards when parked, and placed in a position that no rooftop structures will cast a shadow across them.

    In addition, he will need to park very carefully, or at angles that may not even be possible depending on how he is situated at the site. Seasonal shadow length will also play a large part.

    What I'm trying to say that unles he is willing to watch for shadows, for the most part solar will be a waste of time on the roof unless he does it right. (angled, and possibly elevated above the obstructions).

    He might even be better off doing a portable "ground mount" where he can position and angle them properly during the day, rather than being on the roof.

    AND, here's a common problem which involves shading and just propping panels on a ground mount - some users let grass or other obstructions cast small shadows across the bottom, and not even realize it, cutting their output down to 10% or so.

    Basically, he just needs to be in charge of shadowing. No doubt about it.
    Last edited by PNjunction; 09-24-2018, 05:45 AM.

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