Is this normal or did I gamble and lose?

rgk1rgk1 Solar Expert Posts: 111 ✭✭✭
Saw a local listing for solar panels - 320 watt for $115 each. I bought 4. It was before lunch and the the panels were dirty, but came in at about 38 volts. The VOC is stated as 46.0 one one and 39.7 on the other three. Apparently 3 are 320 watt and 1 was a 325 watt.  Took them home and cleaned them up. Upon initial voltage check, one was about 43v and the others 38-39v just holding them up by myself and trying to get a reading.  There were clouds in the area however none were shading.  Anyhow, I leaned them against the fence with all on a 30 degree angle (I'm located at 45 lat) and it was about 1pm with ambient air temp at 87F and panel temp at 139F. All four came in at 40.5 +/- .3 volts among them. There may have been minimal haze in the atmosphere at the time.

What happens to VOC and/or ISC as a panel gets hot? (or ages, but these do not look old)

What happens to VOC and/or ISC when some cells are partially shaded with dirt/dust?

Is something wrong with them and I simply made a bad purchase or is it normal under these conditions? I really thought the voltage would be higher once I cleaned them off.


4-Risen 320 watt in series/parallel, 4-215ah 6 volt GC2 batteries in series, Exeltech 1100 watt/24 volt inverter, Tristar 45 MPPT controller, Iota 24 volt/25 amp charger.


  • mcgivormcgivor Solar Expert Posts: 3,426 ✭✭✭✭✭
    edited June 2019 #2
    The voltage will drop as the panel temperature rises, the specs are what the voltage should be at 25°C (77°°F) 139°F is 60°C. There is a mathematical coefficient equation  which could be used to calculate what it should be at a given temperature, but if they are all close in identical conditions, that would indicate that every things normal.

    Shading can have a profound effect on a panels performance, even partial shading, if for example the bottom row of cells were shaded, in the orientation of the photo above, the output would be zero. Dirt /dust has some effect on performance, keeping them clean definitely helps, in areas with low rainfall cleaning may be required more frequently.

    The listed values are under perfect laboratory conditions with a new panel, real world values will usually be less, the panels output will decrease in the first few years  then stabilize to some value slightly below new. Solar panels are generally rated for 20 plus years and may  exceed that, some have had factory defects but not many, best to research before purchasing.
    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.
  • mike95490mike95490 Solar Expert Posts: 8,814 ✭✭✭✭✭
    You need to also test the short circuit amps and not arc to the delicate internal contacts of the MC4 connectors.  Output voltage is only part of the test.
    Voltage is controlled by temperature,  hotter = lower voltage
    Amps are controlled by brightness of the light and angle of the panel vs sun.. Brighter = more amps
    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 ||
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  • rgk1rgk1 Solar Expert Posts: 111 ✭✭✭
    That is what I was hoping.  Just didn't know how big a role panel temp played.  I have always read here on the forum that panel output was reduced as the temp increased but didn't recall anything saying how - VOC/ISC or both.  Also just looked at logged data for today on the Kyocera setup and max voltage was 81.4.  Dont recall what time of year I installed them, but at that time and new, I do recall the voltage being 88+.  This weekend I will temporarily substitute 2 of these in place and see what amperage it get. Weather permitting.  For now it sounds like the purchase is sound. Had my eye on some at our host, but dont think i could pay tax and shipping for the price of these.   Thanks mcgivor and mike95490. As a side note, I recently learned the was a large solar installation within 20 miles of me and another being built within 40 miles.  Solar is not the kind of you see around here, let alone two solar farms.  I still get asked if my 540 watt panels powers my house!  Yeeaah - NOT, but i can cook a piece of toast if need be.
    4-Risen 320 watt in series/parallel, 4-215ah 6 volt GC2 batteries in series, Exeltech 1100 watt/24 volt inverter, Tristar 45 MPPT controller, Iota 24 volt/25 amp charger.
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