Solar controller showing low wattage

bmurray15bmurray15 Registered Users Posts: 1
Hello! We’re very new to solar, bought a 12 v system from SunTan Solar. We have two panels that are supposed to be high efficiency, they are also 335 watts each panel. We have hooked everything up, it’s a fairly sunny day, and our controller is showing 350 to 220 watts. Shouldn’t it be showing more as both panels combined are 770? 

Thank you for any input! 


  • PhotowhitPhotowhit Solar Expert Posts: 5,543 ✭✭✭✭✭
    bmurray15 said:
    ....our controller is showing 350 to 220 watts. Shouldn’t it be showing more as both panels combined are 770? 
    What kind of controller and how are you measuring the 350-220 watts? What stage of charging are you at and what type of batteries do you have? Are the panels perpendicular to the sun?

    A lot can be happening, your batteries could be mostly charged and the charging tapering off...
    The panels might be mounted on an RV and not at a perpendicular angle to the sun.

    I took a look at Their solar panel kits and I only see 250 watt panels. Perhaps they had higher wattage panels earlier?

    Solar Panel Kits Archives - SanTan Solar Store

    Something you should be aware of is that solar panels are rated at Standard Test Conditions (STC) but a more realistic output is Normal Operating Cell Temperature or NOCT values. These are about 75% of the STC rating. as the panels warm the cells output somewhat lower voltage and amperage. So perhaps you have 2 - 250 watt panels outputting 350 watts or (350/500=) or about 70% of the STC or about NOCT.  Very light clouds can rob a lot of output.

    If the batteries were close to full they accept less current, Flooded Lead Acid batteries reach this point at around 80% full then accept less current. Maximum current could also be an issue if you have a small battery bank. FLA batteries can only accept 13-15% as they approach that 80% mark. So 350 watts delivering roughly 24 amps at 14.5 volts, would represent 20% charge rate of a 120 amphour 12 volt battery bank...

    So the stages of charging;

    During charging, there are basically 3 stages of charging, Bulk, Absorb, and Float.

    First thing when charging starts you will be in bulk, the voltage rises from what ever the system voltage was to a set point, around 14.5 volts. At that point the Charge controller stops the voltage from rising. Higher voltage can damage sealed batteries.

    Once the battery hits the preset point the charge controller keeps it at that point. Your batteries are roughly 80% full. Flooded batteries will start accepting less current at 80-85% full AGM/Sealed may go a little longer before accepting less current.

    On many controllers you can set this point, Some will have different presets for Flooded, and sealed batteries, or flooded, AGM, and sealed batteries. 

    The charge controller has a couple ways to know when to switch to float, Most inexpensive Charge controller are just timed for 1.5-2 hours. Some will also see less current flowing through the charge controller and shut it down when minimal current is flowing through the controller. On more expensive charge controller. You can set battery capacity to give the Controller a better idea of when to stop. you can also set a longer Absorb time. Or set 'end amps' a amount of amps flowing through the charge controller to stop Absorb and switch to the final stage.

    Once the Controller has determined the battery is fully charged it reduces the voltage to a point where very little current is flowing to the battery. This will prevent the battery from over charging and heating up.

    While in 'Float' the charge controller watch for voltage drop, which would indicate a load. If the voltage begins to drop the charge controller will allow as much current to flow from the panels/array to compensate and maintain the voltage. If the voltage can be maintained, the load will in essence be running directly off the array/solar. If the voltage drops below the preset float voltage, the controller may start a whole new cycle if it stays there for a period of time.

    The system voltage drop you see at night when the sun goes down is the charge controller moving into a resting mode with no energy to contribute to the system.

    The morning voltage may reflect a load present that is effecting the voltage level.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do
  • wellbuiltwellbuilt Solar Expert Posts: 569 ✭✭✭✭
    I can get about half power this time of year . 
     I have 4500 watts of solar and get about 2000 to 2300 max from thanks giving to Valentine’s Day . 
     The sun is low in the sky and far away . 
      I start off with 40 watts in the morning 800ish then by 11am I’m at around 1700/1800 around noon if it a sunny day I can get in 2300 watts . And then the power drops off to nothing at 300ish . 
     That is if the panels are not covered in snow , in reality I have not seen my panels since new years , and there is 5’ of snow on them now 👍 
     The sun really starts picking up now and I get good power for the rest of the year . 
       Or it could be as bill said and the battery’s are getting full and the controller is limiting power .
      A good test is if you put a load on the system your controller should put out max power to feed your load. 
       Try it at now when the sun is best
    Out back  flex power one  with out back 3648 inverter fm80 charge controler  flex net  mate 16 gc215 battery’s 4425 Watts solar .
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