Voc vs Vmpp Panel Voltage

ligwyd
ligwyd Registered Users Posts: 209 ✭✭
edited June 2020 in PV Installers Forum - NEC, Wiring, Installation #1
Voc (Open ciruit voltage)
Vmpp (Voltage at max power point)

When adding voltages to calculate max string voltage going into a charge controller, can you use Vmpp or must you use Voc at all times?
Is it safe to just add up Vmpp and X by 1.25 to account for cold and sunny days? Whats the rule of thumb?

· Share on Twitter

Comments

  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #2
    You always use Voc and adjust it upwards for the historic cold for the site. 

    Generally, the MPPT mfg has a "String Sizer Tool" that your put your panel info into and the min temperature
    and it will report if the proposed configuration is acceptable or if it will violate the warranty.

    In a pinch, you could use this, and massage the results to your device:
    http://www.midnitesolar.com/sizingTool/index.php
      it will at least give you the range of voltages to expect under worst case conditions.

    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

    · Share on Twitter

Categories