Shading calculations for obstructions

BradenBraden Registered Users Posts: 9
So I fully understand and can implement the calculations for interrow spacing to avoid shading for what ever time period for whatever day of the year you chose (and I understand that the industry standard is to run the calculations using the sun's angle and azimuth between 9 am and 3 pm on Dec 21s).

However, I'm wondering if there is an industry standard for calculating shading due to rooftop obstructions. I can run the same calculations (or even model it in Google Earth) for between 9 am and 3 pm on the shortest day of the year. However part of the unshaded space during this time period ends up being shaded at 9 am during the summer (due to the sun's azimuth angle, cause shadows to come off at a much sharper angle during the winter than the summer).

Is there any industry standard, more precise than 2x the height of the object, in determining where to place panels around a rooftop obtrusion. I'm trying to maximize space, but if I place panels on all unshaded areas at 9 AM on Dec 21st, some of those panels end up shaded at 9 am on June 21st.

I'm not sure how well I explained my question, but hopefully someone here can help.


  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Shading calculations for obstructions

    outside of measuring it over the course of a year there isn't anything that could anticipate such abnormal shadowing. i know i have trees to my sw and they shade a bit during spring and fall with a bit in the morning of summer and yet most of those leaves are gone in the winter allowing more light. the branches and trunks do shade too, but would allow far more light than during other times with leaves intact. (you have no idea of how much i've wished like in scifi to have a ray gun and just cut those trees in half.)
  • FrankFrank Solar Expert Posts: 54 ✭✭✭
    Re: Shading calculations for obstructions

    As an aside, one way to minimize impact from temporary/partial shading is to use Enphase inverters (one per panel) rather than a centralized inverter. That means when a particular panel is shaded it doesn't cut output from the entire string.
  • mike95490mike95490 Solar Expert Posts: 9,571 ✭✭✭✭✭
    Re: Shading calculations for obstructions

    Aren't the bypass diodes supposed to allow the array to "bypass" a shaded panel ?
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  • BB.BB. Super Moderators, Administrators Posts: 32,608 admin
    Re: Shading calculations for obstructions

    I believe the bypass diodes are to protect the solar cells if some of the cells/panels become shaded (dark cells go to relatively high resistance).

    The side effect is, if you have a MPPT type controller and a high voltage string (that can withstand 1 or more panels being shaded)--then the bypass diodes + MPPT high voltage string can operate efficiently with some partial shading (as long as Vmp-shaded-array > Vmin-input).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Solar GuppySolar Guppy Solar Expert Posts: 1,987 ✭✭✭
    Re: Shading calculations for obstructions
    mike90045 wrote: »
    Aren't the bypass diodes supposed to allow the array to "bypass" a shaded panel ?


    The Micro inverters will work better only if:

    1) you have shading of a module and it brings the string voltage below the operational minmum of the central inverter when a single string

    2) you have paralleled strings on a central inverter and shade panels in only one of the stings

    My thoughts:

    In general, its dumb to install panels were they will be shaded, its economic suicide

    1) you can lose 25-50% of the panels before you hit the inverter minimum depending on the configuration

    2) You can go to 5kw in a single string now, so the need of parallel stings is not needed for most installations
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Shading calculations for obstructions

    Braden what is your Lat / Long? Closest city?

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  • RCinFLARCinFLA Solar Expert Posts: 1,453 ✭✭✭✭
    Re: Shading calculations for obstructions

    There is no 'cook book' solution as each situation is unique.

    It is often hard to avoid some shading in morning and afternoon. The question is how long does the shading last in relation to unshaded exposure time. At low sun angles in morning and afternoon there would be reduced output anyway even if no shading occurs. The morning/afternoon shading may be more important if a full tracking mount is being used.

    Some panels have multiple bypass diodes which result in only a third or half the panel going into bypass. Even with multiple bypass diodes in a panel the section drop outs will depend on how the shade covers the panel (vertical or horizontal) in relation to how the subpanel bypassing is arranged.

    For a bypassed panel or section section of panel you go from a positive MPP voltage to a negative 0.8 vdc or so for the bypassing diode. The bypass diode can get very hot.
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