Dumb question.. Common mistake?

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machineman
machineman Solar Expert Posts: 129 ✭✭✭
Last week I installed my small 65w solar panel and i used a compass to align it south. Now I realize that's incorrect... I should set it to true south not magnetic south... This is correct right?

Off Grid Cabin, 24V 440ah 6V GC battery bank, Xantrex MPPT60-150 CC, Magnum MS4024 inverter-charger, >1200w Solar bank

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,491 admin
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    Re: Dumb question.. Common mistake?

    You are correct... Magnetic south varies--depending where on the earth you are. In most areas, it probably does not make a huge difference.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.
    BB. Super Moderators, Administrators Posts: 33,491 admin
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    Re: Dumb question.. Common mistake?

    Here is a 1990 chart of the world magnetic declinations:

    http://www.thecompassstore.com/whatisdec.html

    Here are some more detailed maps of various places in the world:

    http://www.geo-orbit.org/sizepgs/magmapsp.html

    And an older page with lots of links and information:

    http://www.geocities.com/magnetic_declination/

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
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    Re: Dumb question.. Common mistake?

    My panels face west, and because of weather conditions, (foggy AM, clear evenings) produce as much, or more, than if they faced south. It all depends on your micro situation. 10 degrees east or west just means you hit peak power a little before noon, rather than just after noon.
    Your mileage may vary
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Dumb question.. Common mistake?

    a good approximation would be to take the sunrise/sunset times and find the time in the middle. that would be solar noon. if you are within 10 or even 20 miles of that tv or radio station giving that info then it's nearly right on. at that time the shadow cast by the sun will be in a north/south direction and you could even get the elevation angle for that day if you're good with math as the shadow length and the height of the shadow source all would help with the angle the sun is at.
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
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    Re: Dumb question.. Common mistake?

    You can find the exact magnetic declination for your location using this calculator:

    http://www.ngdc.noaa.gov/seg/geomag/jsp/Declination.jsp

    The “solar noon” approach can be close, but it’s only accurate twice a year (~ Spring and ~Fall) due to the elliptical nature of the Earth’s orbit around the Sun, the related “Equation of Time”, and the resulting analemma (the Sun’s “Figure 8” path in the sky). For example, here's a link to the analemma for Washington, D.C, a bit east of my home: http://jrscience.wcp.muohio.edu/photos/WashDC1997.gif

    As noted, the alignment just has to be close, and your local climate may dictate some other orientation. For example, if you typically experience cloudy mornings and sunny afternoons, you may want to orient the array a bit towards the West.

    You can also use the PV Watts calulator to model various angle "errors". See: http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/

    HTH,
    Jim / crewzer
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Dumb question.. Common mistake?

    yes i did say it was a good approximation. you are correct that it is accurate during the spring and fall equinox.
  • machineman
    machineman Solar Expert Posts: 129 ✭✭✭
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    Re: Dumb question.. Common mistake?

    I used that NOAA link. Mine is, Declination = 15° 18' E changing by 0° 6' W/year

    Thanks.

    Off Grid Cabin, 24V 440ah 6V GC battery bank, Xantrex MPPT60-150 CC, Magnum MS4024 inverter-charger, >1200w Solar bank

  • Windsun
    Windsun Solar Expert Posts: 1,164 ✭✭
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    Re: Dumb question.. Common mistake?

    Being off a bit will not really make as much difference as you might think.

    What the solar panel sees is a result of the CoSine of the angle - for those (like me) that have forgotten their high school math, just look at a cosine table for 15 degrees and you will see .9659 - which means that the panel will get 96.59% of max power at a 15% angle. It does not start getting serious (over 10%) until you get up over 25 degree angle or so.

    http://www.grc.nasa.gov/WWW/K-12/airplane/tablcos.html
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
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    Re: Dumb question.. Common mistake?
    In most areas, it probably does not make a huge difference.
    10 degrees east or west just means you hit peak power a little before noon, rather than just after noon.
    a good approximation would be to take the sunrise/sunset times and find the time in the middle.
    As noted, the alignment just has to be close,
    Being off a bit will not really make as much difference as you might think.

    I think we have a consensus: It just sort of has to be kind of close! :roll: :-o :-D

    Regards to all,
    Jim / crewzer
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Dumb question.. Common mistake?

    would i be risking an abbot and costello routine if i said "exactly"?