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: 2,652 ✭✭✭✭✭
    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 
    900W  3 × 300W No name brand Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal as a backup system. 
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergencies 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: 28,608 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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