Does this sound correct?

brownbearbrownbear Registered Users Posts: 27 ✭✭
I have two 100w panels, 20amp mppt charge controller and on a sunny day my mt50 shows I'm bringing in 118w.  Does that sound correct?   


  • mcgivormcgivor Solar Expert Posts: 2,834 ✭✭✭✭✭
    edited January 6 #2
    Panels are rated using a Standard Test Conditions "STC" method, this is an artificial light test which establishes a standard which the industry adopts to provide information with which consumers can use to estimate power needs.  Because such a test is in a controled environment with perfect conditions, real world useage may differ. When modules are heated above the test temperature, 25°C/77°F  exposed to the sun for example , the wattage output will drop, when in extreme cold they can actually exceed rated output. Altitude also is a factor, at sea level the output would be less than the same module at 10 000 feet in exact same angle and day due to atmospheric difference.

    Usually a percentage ~80% or 0.8, is used as a multiplier for general calculation, so a 100W module would in most applications will be assumed to produce 80W, however this is if perfectly positioned, the actual maximum will only be available for a short duration each day, as the earth rotates the angle of the sun will change, if plotted on a graph the output would be a logarithmic bell curve. Since seasonal angle changes due th the earth tilting on its axis, a perfectly positioned model on a particular day will only, theroriticly, products it's maximum output twice a year. Most fixed arrays are positioned to achieve maximum harvest when energy needs a the greatest, solar tracking will maximize harvest throughout the year at the expense of complexity.

    So the  short answer is yes, on a particular day it is conceivable 200W STC could  produce only 118W, adjusting to achieve higher output on that particular day will only result in other days of lower output.  
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • brownbearbrownbear Registered Users Posts: 27 ✭✭
    edited January 6 #3
    Two 100w panels, 20amp mppt... Does this mean I can add two more panels to get 245w coming in. Is that a good idea since the mppt can only take 260w at 12v

  • BB.BB. Super Moderators, Administrators Posts: 29,043 admin
    If your two panels are in parallel--This looks to be a working MPPT type charge controller.

    What is/was the battery current/voltage at this time?

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