Solar panel question

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intel415
intel415 Solar Expert Posts: 31 ✭
in Solar Beginners Corner #1
Hi guys, i'm planning to purchase a sharp 240W solar panel from ebay. I want to setup a 12V system to run some lights and one 15v water pump. I have noticed systems go by 12V, 24V and 48V. This 240W sharp solar panel has 15 Amp, 37 Volt open and 30 Volt rated. If i use a morningstar charger to step down the voltage to 12V, where does the rest of the voltage goes assuming if i get 30V on a sunny day or am i just waiting a lot of power on a 12V system? Can someone explain this to me thanks.
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  • solarix
    solarix Solar Expert Posts: 713 ✭✭
    #2
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    Re: Solar panel question

    The short answer is you are wasting the rest of the voltage.
    The long answer has to do with the principle of impedance matching.
    Maximum power transfer occurs when the source (solar panel) impedance (hi voltage/lo current) matches the load (battery) impedance (lo volts/hi current).
    A simple PWM charge controller is going to connect the solar panel to the battery with a big impedance mismatch and lose most of the power.
    An expensive MPPT charge controller will present the optimum impedance to the solar panel and down convert the power to the battery's lower voltage.
    Same principle applies to connecting your car's engine to your rear wheels with a transmission.
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #3
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    Re: Solar panel question

    As Solarix said, MPPT is definitely the way to go. A good MPPT controller, like the Morningstar TS-MPPT-45 (or 60 if you might expand your system in the future) will take the max watts your panels can provide at any given time, and like an AC step-down transformer, drop the voltage to match your batteries, and up the amps. This way there will be very little loss. But with a non-MPPT controller, all you will ever get is the amperage the panels can produce, and all the rest of the otherwise available watts is thrown out the window.
    My cousin installed two such panels last Fall at his camp in the back woods. In his case the panels had to be over 100 feet from the batteries and camp, so the panels were wired in series, giving over 70 volts open/60 rated, which greatly reduced his transmission losses, then fed that into his TS-MPPT-60 and was amazed to see the increased amps going to his batteries.
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  • intel415
    intel415 Solar Expert Posts: 31 ✭
    #4
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    Re: Solar panel question

    Thanks for the great info guys. tristar without digital display is near 500 bucks lol. will a moringstar sunsaver reduce some of the losses? i guess the limiation on this sunsaver is max out is 15A.

    is that why so many people have the 135w solar panel made by kyocera on this forum or they were the least expensive pv? thanks again guys.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,457 admin
    #5
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    Re: Solar panel question

    Yes, for smaller systems a "12 volt" (~Vmp=17.5 volts) does allow the use of a cheaper PWM controller... Plus the 135 watt panel is about the largest that can ship without going truck (although--some folks have gotten larger panels via UPS or FedEx--as I recall--Double check shipping with the retailer--I am not in the solar biz).

    Another controller that should work is the Rogue 30 amps 12/24 volt controller (Vmp on that panel is too low for a 24 volt bank, but should work fine with 12 volt battery bank). The Rogue is a lower maximum input voltage--but would work fine with that panel (or two in parallel)... Double check the Vmp/Imp ratings though--should that panel have Imp~8 amps or so instead?

    The Rogue does include a digital display in the base price (as far as I remember) and is a nice controller (from reviews here).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #6
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    Re: Solar panel question
    intel415 wrote: »
    will a moringstar sunsaver reduce some of the losses?

    Only the MPPT version will reduce losses and give you the use of the available power. But the Sunsavers are limited to 15 amps output to the battery. Your 240 watt panel could, under ideal conditions and assuming no losses (dream on) could drive the MPPT Sunsaver output to 20 amps.
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  • Windsun
    Windsun Solar Expert Posts: 1,164 ✭✭
    #7
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    Re: Solar panel question

    That panel is not rated at 15 amps - it is rated at 8 amps. With a standard controller you will never get more than 8 amps into a 12 volt battery. 12 volts x 8 amps = 96 watts, so you are losing 144 watts.

    Also be aware that shipping on a single large panel will probably cost you around $100 to $150, as they are too large to go UPS.
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  • intel415
    intel415 Solar Expert Posts: 31 ✭
    #8
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    Re: Solar panel question

    Thanks guys for the detailed info. Luckily i haven't pull the trigger yet lol thanks guys.

    @wayne sunsaver the morningstar mppt 15A controller i was talking about. Max 200W charging for 12v system. I will invest in a 45A mppt. Thanks again guys.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,457 admin
    #9
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    Re: Solar panel question

    You can over panel MPPT controllers a bit without "wasting money":

    15 amps * 1/0.77 panel+controller derating * 14.5 volts charging = 282 watt array (rough cost effective maximum array)

    So, roughly, you can hit a 15 amp output MPPT charge controller with upwards of 282 watts of solar array and, most of the time, be at or less than the 15 amp limit for the charge controller.

    If you are in a very cold array with snow on the ground, high altitude, clean panels, etc.--You may actually limit out on the controller more often in the winter. But--overall there is less sun in the winter, so the "missed" power is not that great.

    The MPPT controller itself will not be damaged by connecting to a larger wattage array (as long as you don't exceed the input voltage rating of the controller).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    #10
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    Re: Solar panel question

    If you are going to upgrade , have a look at the Rogue 3024 http://www.roguepowertech.com/
     
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