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Understanding shade and low light conditions

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  • Understanding shade and low light conditions

    Hey All,
    I've been slowly adding components to my tiny Solar PV system. I started with a 12W 12V panel with a cheap PWM controller, it functioned as a glorified cell phone charger.

    A few months ago I got a decent 40A MPPT controller. And just a few days ago I upgraded the panel to 30W 24V. I installed it and ran it in the sun for a few days, it;s working great!. I looked at my charge controller monitor and watched the MPPT do it's thing; it was very exciting!

    Today it's cloudy with rain in the forecast. The panel is producing about 100mA or so, this is more than the old panel would do under these conditions but still pretty low.

    if I were to add a second panel in series I'd get a higher voltage and more current, right? What kind of production can i realistically expect on a cloudy day?

    This leads to my next question. As I further build this out I'll likely build a series parallel setup using 3 arrays; one for morning exposure, one for mid day exposure and one for later in the day, they will all get mid day exposure.

    My question is, let's say I have a single array of 4 panels, what happens to the output if some of that array, say one panel, is shaded?

    I'm just trying to get my head around this.

    12V 12W panel, SolarEpic Tracer4215BN, 12V 18Ah

  • #2
    First off most panels will produce very little charging amps on a cloudy day. Even if you wired 10 of those panels you might only get 1 amp total.

    Next item is that if you have a string of panels wired in series then shade on even one of them will reduce the output of the entire string.

    You might as well forget about harvesting solar power on cloudy days. The cost of increasing your array wattage will far outweigh other emergency power generators.


    • #3
      Thanks SunEagle! I figured as much. Now I want a wind turbine...
      12V 12W panel, SolarEpic Tracer4215BN, 12V 18Ah


      • #4
        Originally posted by Richiep View Post
        Thanks SunEagle! I figured as much. Now I want a wind turbine...
        Well, most wind turbines are plain junk unless you have continuous wind blowing above 15 mph. You also need to have the turbine way up in the air with about 200 feet clearance 360 degrees from trees and buildings around it.

        Don't get fooled by those small wattage, blade diameter units. They won't produce anywhere close to their nameplate wattage.


        • #5
          The response of a series parallel array to shade will be complicated. It helps if you think of each individual panel as 60 cells (or whatever) in series. When you put two panels in series, it isn't very different than putting 120 cells in series. As in any series circuit, the current is constant throughout the circuit. If one cell isn't capable of generating as much current as the others in series, it will limit the output of the whole series to just the current it can generate (and it might get very hot!).

          To mitigate failure of a single cell, or to deal with partial shade, most solar panels include bypass diodes that offer an alternative path for current to flow around each series of 20 cells. The diodes are designed such then when a cell fails or becomes locally shaded, the voltage bias reverses and current can pass through the diodes. Effectively, the 60 cell panel would become a 40 cell panel.

          In a single series string, the voltage of the string drops as each 1/3rd of a panel ceases to contribute to voltage production, but it isn't worse than that.

          If your panels are in parallel, partial shade cause cause 1/3rd of a panel to be bypassed, and because that panel is now operating at a lower voltage, it could have an impact on the other unaffected panels (if you are using an mppt charge controller).

          In the case where you have panels facing different directions, that isn't really a "partial shade" situation in which the bypass diodes will get involved. In that case, you would definitely want each orientation to be its own string, and have those strings be the same length, all connected in parallel. As long as the light on with each orientation is uniform, they will all be at about the same voltage, and happily produce whatever current they can without a care about whatever is going on with the other orientations.
          CS6P-260P/SE3000 -


          • #6
            Here are a couple of videos that I thought were pretty good.




            • #7
              Thanks for the detailed response Sensij it was a great to read before looking at the excellent videos that AzRoute66 posted.

              I feel like I now have a handle on shading and low light.
              12V 12W panel, SolarEpic Tracer4215BN, 12V 18Ah