max number of panels per string

momofgcmmomofgcm Solar Expert Posts: 35
Is there a general design guideline for the max number of panels that can be put in parallel to the controller?

How about how many panels in a series string when hooked in series parallel?

Kim

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: max number of panels per string
    momofgcm wrote: »
    Is there a general design guideline for the max number of panels that can be put in parallel to the controller?

    How about how many panels in a series string when hooked in series parallel?

    Kim

    For any given controller the answers would be:

    In parallel, up to the number that will "max out" the controller's output capacity in Amps. Id est; for a 60 Amp controller on a 12 Volt system approximately 720 Watts of panel (Amps * Volts). The real number is dependent on the panel's actual power capacity. Whereas you can put more panels on than that, it would really be just wasted power because, in general, the maximum output of the charge controller is the limit.

    In series, up to the number that will "max out" the controller's input capacity in Volts. This can be trickier to come up with, as panel output Voltage can go higher than specified under certain conditions (most notably cold) or be significantly lower under others (hot and sunny). For PWM controllers, the 'nominal' panel Voltage shouldn't exceed the 'nominal' system Voltage, as in a 12V panel (whose actual Vmp may be 17 Volts or more) on a 12V system (even though actual charge Voltage will be approximately 14.2).

    There are some subtle differences between specific controllers, but in general terms those are the limits.
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: max number of panels per string

    It will all depend on the specs of the controller. Generally it is amperage that will over-limit first for parallel and voltage that over-limits for series configurations. Though with enough panels you can over limit both.

    ie a series string will have the panel voltages accumulate and amperage stays the same for the string.
    In multiple strings (parallel ) the amperage accumulates...

    HTH,
    EJ
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
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  • BB.BB. Super Moderators, Administrators Posts: 32,606 admin
    Re: max number of panels per string

    There is a major difference between PWM (the inexpensive controllers) and the MPPT type (2-3x as expensive).

    MPPT controllers will limit input current based on the controller's needs and abilities---You can safely add more panels (wattage) than the controller can support (within the limits of the controller, wiring, fusing/breakers). The controller has the ability to control current flow internally (switching power supply/down converter).

    PWM controllers are basically a switch that turns on or off (can be 100's to 1,000's of times per second). There is no inherent current limiting capability, so it is very important that you respect a PWM controller's input current limit--I think the NEC requires a 1.25*1.25-1.56 margin of safety between Imp/Isc and controller input current limit (may be a bit excessive--but certainly safe).

    You have to be very careful about rules of thumbs and design limits--there are a lot of different types of solar equipment out there--and they are no longer "brain dead simple"--but very sophisticated computer controlled power conversion gear.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: max number of panels per string

    Bill makes a great often overlooked point.

    My controller has a maximum capacity of 30amps. I have deliberately installed more PV such that the combined total might exceed 30 amps. In that event, the controller limits the output to 30 amps. The reality is that there are very few times in the year when the panels will put out that much,,, usually more like 25 amps. The point is the MPPT controller can do that safely, but a PWM can't.

    As a result, I can make maximum use of my controller and my panels nearly all the time.

    Tony
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