solar panel voltage

rake1
rake1 Solar Expert Posts: 173 ✭✭
in Solar Beginners Corner #1
I am looking at adding another 12v panel the ones I have now are 125w ,135w and I want to add another 135w. Two panels are 12v-17.5vvmp the new one I believe is 12v-20vmp What is the difference in the vmp mean and will they work together.
· Share on Twitter

Comments

  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #2
    Re: solar panel voltage

    Yes they'll work together if they're all in parallel- they won't be working at the maximum power, but they will be working.
    Your 125W panel with a Vmp of 17V will have an Imp of 7.3A.
    The new 135W panel with a Vmp of 20V will have an Imp of 6.7A.

    Assuming you're using a PWM controller then:
    When the battery is at 12V, then the 125W panel provides: 12v x 7.3A = 87W and the new 135W panel would provide: 12V x 6.7A = 80W.
    So you'll be making less power from the new 135W module than from the old 125W.
    · Share on Twitter
  • rake1
    rake1 Solar Expert Posts: 173 ✭✭
    #3
    Re: solar panel voltage

    So I understand now a 135w with 17.5vmp would be 7.7 amp correct?What is the benefit of the higher VMP? I thought the more amps the better but the higher vmp must have a benefit. Also why do you say they won't be working at the maximum power? Is that because of the higher VMP?
    · Share on Twitter
  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #4
    Re: solar panel voltage
    rake1 wrote: »
    So I understand now a 135w with 17.5vmp would be 7.7 amp correct?

    Yep.
    rake1 wrote: »
    What is the benefit of the higher VMP? I thought the more amps the better but the higher vmp must have a benefit.

    Unfortunately there's no benefit to higher Vmp if you're using a PWM controller. Now if you have an MPPT controller then it isn't limited by the battery voltage and can take full advantage of all the power available from the panel.
    rake1 wrote: »
    Also why do you say they won't be working at the maximum power? Is that because of the higher VMP?

    Yes, the 135W panel will only ever produce 135W when the voltage is 20V (Imp would be 6.7A). Since your battery will never reach 20V, the panel will never produce it's maximum power. At most when you EQ the battery at 15V, it may produce 100W (at standard test conditions).
    · Share on Twitter
  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #5
    Re: solar panel voltage

    Double check the specifications too... It is possible that the panel rating is Voc~20 volts -- And it may turn out that Vmp~17.5 volts, which would "match" your exiting panel better.

    More or less, you need a minimum voltage from the array (Vmp--voltage maximum power, Imp--current maximum power) because deep cycle batteries (plus voltage drop for wiring and controller) can exceed 15.0 volts and the output of the solar panel drops several volts on very hot/sunny days (Vmp/Voc fall with increasing temperatures).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
    · Share on Twitter
  • rake1
    rake1 Solar Expert Posts: 173 ✭✭
    #6
    Re: solar panel voltage

    checked it it is 18.2v thanks
    · Share on Twitter

Categories