Solar panel watts&amp output

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Jerry17
Jerry17 Registered Users Posts: 3 ✭
edited January 2017 in General Solar Power Topics #1
Hi,
 I have 2 130 watt panels on my RV for a total potential of 15amp output per hour if my battery is low in the morning.
A friend recently installed one 260 watt panel on her van that is rated for about 8amps output per hour.
 Can you explain this?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,696 admin
    #2
    It is a bit more complicated that that... Assumign your friend has a MPPT type solar charge controller--they are doing just fine.

    You probably have a PWM charge controller--Which is fine for your installation

    Power = Volts * Amps.

    A PWM controller "lets through Amps"... I.e., (forgetting losses), 15 amps in is 15 amps out.

    A MPPT charge controller is a "down converter"... A DC switching (buck mode) power supply. Again, ignoring losses, the MPPT controller takes Watts in and outputs the same Watts out:
    • Power = Voltage * Current; Current = Power / Voltage
    • 260 Watt / 14.5 volts charging = 17.9 amps out
    • Including losses: 260 Watts * 0.77 panel+controller deraiting * 1/14.5 volts = 13.8 amps typical maximum current
    A PWM controller:
    • 260 Watts * 1/18 volts Vmp = ~14.4 volts Imp (current maximum power)
    Realistically, if both installation are properly designed and installed, they have about the same output.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Jerry17
    Jerry17 Registered Users Posts: 3 ✭
    #3
    Thanks for that explanation. I had been faked out by the 8 amp rating on the label while misunderstanding the different types of controllers. I had kind of thought the panel might have been 24v but anyway... last thing I needed to know. heh.
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  • Jerry17
    Jerry17 Registered Users Posts: 3 ✭
    #4
    Hi,
     Well, I found out now that her controller is a PWM Sunsaver 20 by Morningstar. So, she is getting only the 8amp rating to the battery?
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  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,134 ✭✭✭✭✭
    #5
    Yes, and that 260 watt panel is putting, at best 115 watts into her batteries.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,696 admin
    #6
    This is a good small MPPT controller (get the remote battery temperature sensor option) for your friend (the 260 Watts of panel is about the largest that this small controller will run on a 12 volt battery bank--Would need a larger controller if more panels added later:

    https://www.solar-electric.com/mosumpsochco.html ($200 + RBTS cost)

    There are a lot of larger MPPT controllers to pick from out there...

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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