Need help choosing correct solar panel

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  • EstragonEstragon Registered Users Posts: 3,034 ✭✭✭✭✭
    Sorry, it's short for PhotoVoltaic solar panel.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • myocardiamyocardia Solar Expert Posts: 118 ✭✭✭
    So a 20w solar panel would put out around 1.25amp given the 75% figure you just gave  me? That would be 1.66amps x 75%= 1.25 amps per hour from the 20w solar panel if i'm correct?  Down here in Texas we get anywhere from 8-10hrs of sun on a good day, or normal day.

    No, not at all. Solar panels aren't rated at 12.0V, they are rated at the voltage at which they output their highest amount of power. For 12V panels that are worth owning, that will be equal to or higher than 17.0V, assuming we are talking about current panels, not stuff from the 1970s, 1980s, etc. The cheaper panels, what we call "poly" (short for polycrystalline, but are also called multicrystalline) are pretty close to 100% of the time rated at 17.0V to 17.5V. Divide 20 watts by 17.0V, and you get 1.17A. Divide 20 watts by 17.5V, and you get 1.14A.

    That 1.17A is what you will end up getting 75-77% of, before wiring losses, and before charge controller losses, and before battery charging losses. That means that a 20 watt @ 17.0V panel can only give you 0.8775-0.90A for an hour per day, at solar noon. And since there are so many losses, as outlined above, you will be able to put 50-52% of that 0.8775-0.90A into your battery, during that one hour per day. You'll get enough power to put some charge into a battery from it for longer than that, but not for much more than 7 hours per day, and only that much in July. Chasing the sun helps get a few more hours per day of charging, if you have unmounted panels, such as your 20 watt panel.

    DoD= depth of discharge= amount removed from that battery   SoC= state of charge= amount remaining in that battery
    So, 0% DoD= 100% SoC, 25% DoD= 75% SoC, 50% DoD= 50% SoC, 75% DoD= 25% SoC, 100% DoD= 0% SoC
    A/C= air conditioning AC= alternating current (what comes from the outlets in your home) DC= direct current (what batteries & solar panels use)
  • LumisolLumisol Registered Users Posts: 374 ✭✭✭
    You could use one of the flexible unbreakable panels and not worry about broken glass around the animals if you wanted.
  • BB.BB. Super Moderators, Administrators Posts: 28,176 admin
    I am not a fan of flexible panels. Perhaps they last 5-10 years (imho).

    And you do not want flexible panel fragments getting near any animals or food sources. Some thin film panels can contain cadmium and/or other heavy metals.

    If anybody has experience with long life flex/plastic panels, please feel free to post.

    - Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • LumisolLumisol Registered Users Posts: 374 ✭✭✭
    I never saw any plastic ones, that's a bad idea all around and should be avoided. I have only seen the long life materials used personally.
    Glass in a pen is a bad mix too. It isn't toxic but can do a lot of physical damage and cause internal bleeding.
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