Off Grid Home Forums Technical Discussion Solar panel effectiveness

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    Not all solar panel arrays are created equal. From my days of working at Xantrex I knew this but had not put exact numbers to it. I am currently located at latitude 53 degrees 30 minutes north. Sunrise is around 5 AM but the sun rises in the northeast and sets towards the north west. With a solar panel array set up to face south the edge of the panel would face directly east and west. In other words the sun rises BEHIND the panel and sets behind it in the evnening. Keeping track of the shadow I determined that at nine AM the sun falls on the edge of the solar panel and is due west by mid afternoon. Usable sunlight only strikes the face of the panel from about ten AM to about three PM for a total of no more than five hours and only the middle three hours actually produce any real output for charging my batteries. By compariosn during the short winter day I see sunlight striking the face of the panel at a workable angle for maybe 4.5 hours . Unfortunately during the winter months the sun is so low to the horizon the strenght of the sun is far less effective than during the summer months when the sun is higher in the sky.

    I do know that some models of solar panel will deliver a usable charge due to ultraviolet days but not all of them do. I also know that some panels are very sensitive to angle of incidence. My tests with panels on a tiltable and rotatable test stands clearly show a tracking mechanism is highly desirable. Unfortunately building steerable arrays is very expensive. Especially if the panel array is large and weighs a lot. It requires big support bearings and more powerful tracking drive motors.

    If I did some detailed calculations I would probably find the actual duration of charging output is even less than the numbers above would suggest. Even an MPPT controller can’t improve on incident angle on a panel. If the sun is behind a solar panel it is not going to produce an output. What this means is as you locate further away from the equator the size of the required panel has to be increased in order to maintain a proper charge to the battery bank needed to drive your particular household. If two solar panels is sufficient to charge a given size battery bank way down south farther north you may need three or four panels to equal the same output. This makes the cost per watt even more expensive. Even the best solar panels are only about 20% efficient at the best of time .

    Moving them away from the equatorial regions simply agrevates this. Based on actual output current I have determined my silar panels are about 10% efficient. A national chain store sells a panel rated one watt and this panel barely produces half a watt at noon in the summer.

    To my surprise I found the solar panels used in the decorative garden lights to be more efficient . They deliver the same output over a wider angle of incident light. However the output is consistently lower than the other cells. To match the purpose built panels intended for larger battery charging these simply would not be very good. It would require too large a surfact area .


    Yes. Positioning is important to solar panels, but that’s not the only factor. The sun’s azimuth (as elnav pointed out) is an often overlooked factor. I live at latitude 47 degrees 88 minutes north. That’s pretty far North for solar collectors. PV and solar collectors will always do better the closer to the equator.

    Tracking panels are available, but cost big bucks. The successful solar panel installations that I have seen up here have either been on the bank of a lake, or in the middle of a field/pasture (about 3/4 acre), and I know one installation that uses a reflector setup to direct sunlight light into the collectors.



    I wondered about using mirrors. Spain has a solar thermal park where they use sunlight to melt sodium for heat transfer driving a steam turbine but I wasn’t suer this was practical for small single house installations.


    My first system was limited by roof size and orientation of the original house. A shell I finished, all trades. The only way we could afford another house with wages suppressed for 20 years by the invasion.

    I calculated the need for 11 -75W panels without tracking. So I bought a Wattsun and used 8 panels for the same power. Maintenance was done under warranty once, and I bought a $200 manual control because the sun seeker would pancake the array at times of hail or heavy snow.

    When I expanded the house I found that using Unisolar amorphous film panels at just the latitude angle, fixed with simple 1×1 aluminum Ls and bolts gave around 90% of the power per watt as the full tracking array. Plus were unbreakable.

    So with the additions we increased our power 60% for 30% more money, roof mounted. Remember, all do it yourself, too.

    Now there are more textured front unbreakable thin film panels available. When I had to fix my glass array at the same angle as the Unisolars to send in the sun seeker for repair, the reflectivity of the glass with the sun at a 45* angle to the panels gave only 10% power, while with the Unisolars it gave 90% power. So for beginners and less expense, I would certainly recommend the textured surface unbreakable amorphous film type panels over the smooth glass panels. At least for 39* latitude or so, or less.

    Of course, at the extreme north it still is a good idea to track for best performance.

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