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BFL13's avatar
BFL13
Explorer II
Mar 21, 2017

Panel Wattage Rating Question (Midnite Mystery?)

I am confused about how they do the panel wattage at 12v vs 24v for a controller.

EG, the usual blurb seems to be that output wattage rating for an MPPT (EG Morningstar 60 amper is 800/12v and 1600w/24v for your array.

I don't understand why they have that for output when the array is input.

However, I gather ?????? that the controller has the same amps limit (self -limiting) to 60 amps no matter what the battery voltage is, so if your battery system is 12v then you get 12 x 60 = 720w as your limit (800 here, so amps limit must be a touch higher?) and with 24v system, you get to have 24 x 60 = 1440W of panels except they allow 1600w.

Similar with many other controller brands which have twice the panel wattage allowed for 24v system (meaning battery voltage AFAIK) than for 12v systems.

Then we get this Midnite Mystery--
http://www.midnitesolar.com/pdfs/spec_sheet_classic.pdf

Where the output amps is less for higher voltage systems. So what is the rating for panel wattage at different voltage systems? You can't double the watts if the amps rating of the controller does not stay the same going from 12 to 24 eg.

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  • CA_Traveler's avatar
    CA_Traveler
    Explorer III
    Mar 21, 2017
    BFL13 wrote:
    I can see how the wattage doubles if you hold amps the same and double the voltage. But that Midnite has lower output amps ratings for each rise in voltage. I didn't see where they show any panel wattage amounts.
    So what? These are very sophisticated electronic designs that are NOT linear. Midnight is likely the top dog of CCs and I wouldn't doubt their specs for a second.

    Don't know why you do. :(
  • BFL13's avatar
    BFL13
    Explorer II
    Mar 21, 2017
    The MS 60 has a "nominal" panel watts limit of 800/12 and 1600/24, but does allow for "over-panelling" because it has that 60 amp output control. 800w/12v = 66.7 amps. They have a note suggesting over-panelling can be good for you.

    I can see how the wattage doubles if you hold amps the same and double the voltage. But that Midnite has lower output amps ratings for each rise in voltage. I didn't see where they show any panel wattage amounts.

    if you do have 800w rating of panel as input, then with 25C above ambient for panel temp, like 25c/50C then you have 10% power loss: 800-80=720w and
    720/12 = 60 amps but you can have other circumstances.
  • CA_Traveler's avatar
    CA_Traveler
    Explorer III
    Mar 21, 2017
    BFL13 wrote:

    If at 14.8v battery and the output watts limit is 800w can you ever see 60 amps? I think it would be limited to 800/14.8 = 54 amps but I don't own one to have noticed that.
    The MPPT60 is limited to 60A. There is no stated input/panel watt limit.

    It's clearly stated that if the panel watts should result in exceeding 60A output the CC will derate to 60A.
  • MrWizard's avatar
    MrWizard
    Moderator
    Mar 21, 2017
    Are we taking about pwm or mppt
    For Pwm I chose to go by total amps not panel watts
    There is voltage drop, not amperage drop I calculate Panel max amps times 14.xx
    And use that for max input watts

    But for mppt, the controller does volts to amps conversion, so I would use the max watts rating of the controller listing

    I am on my tablet and did not read the minnite solar pdf
    Might be nothing more than a typo
  • CA_Traveler's avatar
    CA_Traveler
    Explorer III
    Mar 21, 2017
    BFL13 wrote:
    Controller efficiency is better at 48/48 than at 48/12
    Does Midnight state this?

    Regardless the CC may have to limit the amps for reasons other than efficiency.
  • BFL13's avatar
    BFL13
    Explorer II
    Mar 21, 2017
    Controller efficiency is better at 48/48 than at 48/12 so why is there a lower amps rating for the Midnite at 48v (battery voltage, I ASSume) than at 12v?

    If at 14.8v battery and the output watts limit is 800w can you ever see 60 amps? I think it would be limited to 800/14.8 = 54 amps but I don't own one to have noticed that.

    My Tracer 2210A MPPT manual says its "rated charge power" is 260w/12v or 520w/24v and that the controller will limit charging current to the "rated charge current" (20 amps) However, 260/12 = 21.7 amps

    Is the self-limiting based on the amps rating or the input power rating?

    It says you can over-panel but at three times "charge power" you will fry the controller. So max panel allowed for my 20 amper is 3x260= 780/12v.

    But panel is input and charge power is output.
  • CA_Traveler's avatar
    CA_Traveler
    Explorer III
    Mar 21, 2017
    The numbers you are using are mostly for ball park planning.

    For example the MS MPPT 60 has a limit of 60A. So for bulk charging at 14.8V the watts to the BATTERY are 14.8*60 = 888W. And the CC input watts have to account for the controller efficiency. And the array watts have to include wiring losses.

    The Midnight numbers might be a reflection of the almost 2x watts that are involved for 24V vs 12V - think electronics heating and efficiency. It might even involve the efficiency of cooling the electronics with the fan.
  • 2oldman's avatar
    2oldman
    Explorer II
    Mar 21, 2017
    BFL13 wrote:
    You can't double the watts if the amps rating of the controller does not stay the same going from 12 to 24 eg.
    You sort of have a double negative there, but

    10amps x 12v = 120w
    10amp x 24v = 240w.

    I have no idea if that's your question.

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