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  • Solar controllers

    I read an old reply to an old post on this forum from a few years ago I think. It was very informative (thanks SunKing), but there were several mentions of MPPT controllers and their efficiency. I think it was stated in an example calculation that the efficiency of these controllers was at best 60%. Since then, I have read and been told that it may be closer to 90%. That being said, I have also been told that I really don't need a MPPT controller for my purposes (I have a 100 watt suitcase style solar panel from Renogy to use for charging the battery in my tiny teardrop trailer. I don't use much power right now, but that will likely change in the future when I upgrade the vent fan from a measly 1.5. amp to a 3.0 amp max. I I know it's still not much, but I but the 7.4 Ah battery I have now will not run even the 1.5 amp fan through the night. My calculations led me to believe that I will need a 45 Ah battery for the new fan and that my solar panel should be able to recharge it in a couple of hours from 50% discharged. I'm willing to admit these numbers may be wrong of course and welcome any corrections. So, question is, would it be worth it to get a MPPT controller or would a much cheaper 30 amp PWM controller work just fine for me? I like and appreciate the higher efficiency, but they do seem to be much more expensive pound for pound. I am also not sure of the PWM controllers efficiency, but I assume it's no where near 90% or even 60%. Thanks in advance.

  • #2
    The numbers given are a bit off. MPPT controllers can be up to 96% efficient, while PWM controllers are probably more like 70%,
    In your case, the cost of a real MPPT controller would be prohibitive. They need higher voltage than PWM controllers which would require you to buy another matching panel, at least. I have never seen a MPPT controller smaller than 10 amps and IMO the only reason to use one this small would be if you had a high voltage/low amperage thin film panel. You need somewhere around 250 watts of panels, minimum, to begin justifying the increased cost of a MPPT controller.
    2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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    • #3
      Thank you. That's actually great news. First and most importantly to me, it confirms what I have heard. Second, you give a great explanation, which I hadn't heard and definitely appreciate. Third, it's much cheaper for me. Forth, PWM controllers are evidently way more efficient than I feared. Is there a way to tell how efficient each different PWM controller is?

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      • #4
        Originally posted by Ianssun View Post
        Thank you. That's actually great news. First and most importantly to me, it confirms what I have heard. Second, you give a great explanation, which I hadn't heard and definitely appreciate. Third, it's much cheaper for me. Forth, PWM controllers are evidently way more efficient than I feared. Is there a way to tell how efficient each different PWM controller is?
        It would be hard to list each and every PWM model along with their posted efficiencies.

        If I were you I would read all of the manuals I could find to see what they state although it is not uncommon for some manufacturers to make false claims concerning their product.

        That is where you start to weed out the cheap crap and focus on the quality name brand equipment. While you will pay more you have a much better chance of getting a product that meets your needs as well as the stated specifications.

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        • #5
          PWM controllers work by amperage in = amperage out. Whatever voltage above the battery voltage will be lost. An example would be if you had a 24 volt 175 watt panel to charge a 12 volt battery. The panel would put out about 5 amps @ 35 volts. With a PWM controller you will get 5 amps at say 13.5 volts =67.5 watts. All voltage above the battery is lost, With a MPPT controller you will get 12,96 amps, which is 175 watts divided by 13.5 volts. The MPPT controller will step down the higher voltage, while boosting the amperage. There are some losses due to not being 100% efficient and real world conditions, but that's why, in a nutshell, MPPT controllers make such a big difference. So as far as efficiency of a PWM controller goes it depends what voltage you are feeding it in relation to what voltage the battery is pulling from it. If you have a solar panel that constantly put out 14,8 volts it would be quite a bit more efficient than my example. 12 volt solar panels put out about 17 to 18 volts under load and that over voltage is lost. The problem is when solar panels get hot the output voltage drops so a panel rated at 14.8 volts in the lab will put out too low a voltage in real world conditions and not have high enough voltage to actually charge a 12 volt battery which needs to be charged up at 14+ volts to be able to "force" power into it.

          I may get corrected here but that's how I see it. Sorry if that seems a little long winded.
          Last edited by littleharbor; 06-06-2017, 09:38 AM.
          2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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          • #6
            Originally posted by Ianssun View Post
            Thank you. That's actually great news. First and most importantly to me, it confirms what I have heard. Second, you give a great explanation, which I hadn't heard and definitely appreciate. Third, it's much cheaper for me. Forth, PWM controllers are evidently way more efficient than I feared. Is there a way to tell how efficient each different PWM controller is?
            The efficiency of a PWM controller is based more on the voltage of the panels vrs battery voltage then on the controller. The variation from PWM to PWM controller is likely less than 1%.

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            • #7
              Originally posted by Wy_White_Wolf View Post

              The efficiency of a PWM controller is based more on the voltage of the panels vrs battery voltage then on the controller. The variation from PWM to PWM controller is likely less than 1%.

              WWW
              Maybe and maybe not. Some of the really cheap PWM CC's may have a much lower efficiency standard based on where they are manufactured.

              IMO a blanket statement of < 1% difference is too general to make.

              The explanation that littleharbor provides is pretty good rule of thumb when selecting a PWM CC. The total efficiency is also a product of the type of solar panel used and its Imp rating.

              Since a PWM is Amps in = Amps out a lower Imp will generate much less charging amps on a lower voltage battery system.
              Last edited by SunEagle; 06-06-2017, 10:12 AM. Reason: added last sentence

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              • #8
                Your confusion is understandable and caused by not understanding the big picture.

                At worse a MPPT system is 67% efficient, and at best PWM is 50%.

                Here is the big picture, MPPT controller is 92% or better efficient input to output. Bu that is only one part of the losses incurred in a SYSTEM you are not accounting for. You have wiring losses, battery Charge Efficiency and Inverter efficiency on top of that you are not accounting for. Add them all up and you are loosing 30+ percent of power.

                MPPT OUTPUT CURRENT = [Panel Wattage - Converter Efficiency] / Battery Voltage
                PWM Output Current = Input Current

                A 100 watt panel has a Imp = 5.55 amps and Vmp = 18 volts. So let's say we use a 97% efficient MPPT controller.

                MPPT Output Current = [100 watts - 3 watts conversion] / 12 volts = 8.1 amps = 97 watts
                PWM Output = 5.5 amps = 66 watts
                MSEE, PE

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