Why do my panels work better when it's cloudy?

james2170james2170 Registered Users, Users Awaiting Email Confirmation Posts: 1
edited May 2018 in Solar Beginners Corner #1
Hi I have a strange problem so I wonder if anyone has any ideas, under complete cloud cover my panels are pulling 130W but when the sun comes out and with a completely clear blue sky and strong sunlight they drop to around 40-50W! This is consistent daily. I am baffled. Readings are from the LCD on my MPPT charger.

Have tried all sorts of angles on the panels but they seem to be in the correct position and the sun is hitting them. Not only this but also puzzled as to why my input is so low given I have 4 huge panels. This will be my next problem to solve. I've triple checked all my wiring but I'm open to any ideas I'm guessing I must have something wrong somehow.

My setup is 4x265W panels in series connected to an MPPT charge controller and 4x12V batteries in series (48V). I have nothing causing a shadow over any panel. I can add photos if it helps. I have tried a couple of different charge controllers and both produce the same result.

I am getting around 140V from all 4 panels at both times (under sun and then under cloud). Actually under sun the voltage goes up to around 150V sometimes but not over. The charge controller operates from 70V to 170V according to the manual so I am within the range and no alarms are on the controller so it seems happy.


  • mcgivormcgivor Solar Expert Posts: 2,500 ✭✭✭✭✭
    edited May 2018 #2
    Seems strange, what battery voltage is displayed? If the batteries are close to fully charged the current would diminish even in full sunlight.
    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.
  • EstragonEstragon Registered Users Posts: 3,127 ✭✭✭✭✭
    It does sound like some sort of shading issue, where diffuse light from cloud affects output less than a well defined shadow in full sun. Is the anything (wire, vent pipe, bird bomb, leaf, etc) on or near a panel that could be causing even a very small shadow?

    If not, I'd try removing 1 panel from the string at a time (cover panels with a blanket first to avoid making/breaking connections under load). If you get good production with a string of 3, the panel taken out may be defective or damaged.

    As Mcgivor notes, the battery will have to be partly discharged so there is a load on the string for testing.

    I'm assuming the controller has the required capacity to handle the output current (eg 30a or more).
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • VicVic Solar Expert Posts: 2,938 ✭✭✭✭

    Hi James,

    What MPPT Charge Controller is this system using ?

    Thanks,   Vic

    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • mike95490mike95490 Solar Expert Posts: 8,067 ✭✭✭✭✭
    It's possible that the controller is unable to work with the low shaded power.
    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
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  • NANOcontrolNANOcontrol Registered Users Posts: 96 ✭✭
    Maybe the batteries just charge faster and you are only looking at float current. Under cloudy conditions you are drawing power for a longer time and it is more noticeable.  That is why I went to heating water with excess PV, Not to just waste power when not needed for charging. I get all the hot water I need with no change in the rest of the system.
  • terracoreterracore Registered Users Posts: 3
    Sounds like your batteries are quickly topped off under full sun and you are only seeing the float.  Try putting a heavy load on your system and see if your readings change.  If that power has nowhere to go, you'll see lower numbers like you are reporting.  Also make sure your charge controller is programmed for the type of battery you are using (flooded, sealed, AGM, etc).
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