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  • Newbie with RV calculations

    Hello Everyone,
    Mi name is Maxi. I'm from Argentina. I'm starting to plan my rv solar system.
    For now I only use a battery wich I previously charge to use the lights mainly.
    I'm learning a lot from the forum.
    I've started reading the main stickys by sunking and other people's post, and made the next calculations:

    following post "solar off-grid battery design"

    4 led lights 12 watts x 5 hours = 240 watts
    1 water pump 90 watts x 2 hours = 180 watts
    3 cell phones 10 watts x 2 hours = 60 watts
    1 LCD TV 50 watts x 3 hours = 150 watts
    1 directv deco 50 watts x 2 hours = 100 watts
    1 notebook 60 watts x 3 hours= 180 watts

    total in a day 910 watts x 1,5 factor= 1365 watts in a day (1400 to round up)

    Hours of insolation 5

    Size of array: 1400/5= 280... round up 300
    battery capacity 341 amp (3 day reserve)
    Charge size: 25 amp (30 mppt regulator)

    I will also have a generator to charge full twice a week batteries.
    Thank you for checking my numbers.
    Maxi
    Last edited by maxgarde; 05-15-2018, 10:28 AM.

  • #2
    You might be over estimating how many sun hrs you are getting. Remember that you want to base it on the month with the lowest solar insolation and not the yearly average. In Argentina that should be between May-August.

    Before you do all that invest in a good battery isolator so that your RV can charge your batteries while you drive.

    Comment


    • #3
      Hey Maxgrade welcome. I am the one who wrote the tutorial so allow me to make few comments and suggestions. Puedo ahorrarte mucho dinero. Bueno?

      First is minor detail and does not change anything. Your terminology is wrong and you are confusing Watts with Watt Hours. Watts is power or heat, and the rate at which energy is being consumed. Watt Hours = Watts x Hours, is energy consumed over some period of time. Example a 100 watt light bulb will consume 1000 watt hours in 10 hours, 100 watts x 10 hours = 1000 watt hours. Your math is OK for watt hours, but you did not specify any battery voltage or AH capacity.

      Next is the tutorial is written assuming a USA location in a fixed structure on a full time basis. You are not in the USA or in a fixed location. In addition changes panel sizing to minimum required to support the battery. You do not have 5 Sun Hours, but no big deal. With such excellent location and using your Solar Insolation, the calculations can come up with Under Sized panel wattage to meet minimum battery charging requirements.

      OK lets talk battery. Being in an RV you can get away with 3 day reserve capacity because of infrequent use, that still gives you 2 days run time before you must shut down and recharge. You can still run 5 day reserve if you wish and can afford the weight penalty and have space. Just be aware you have an extra option being in an RV.

      Battery AH Capacity = Days x Daily Watt Hours / Battery Voltage

      Assuming 3 days, 1400 Watt Hours, and 12 volt battery:

      3 x 1400 / 12 = 350 AH or 583 AH @ 5 day reserve. Your wallet will make that decision make the decision, not you when you determine panel wattage and controller size.

      OK with 5 Sun Hours your panel wattage will be too low to generate a C/10 minimum Charge Current, especially whit 5 day reserve. C/10 is where C = the battery AH capacity divided by a digit representing Hour Charge Rate. So if you have a 250 AH battery means you need at least 35 amps charge current if you depend on solar to do daily charging. That changes the Formula for Panel Wattage = Charge Amps x Battery Voltage. So a 350 AH battery is 12 volts x 35 amps = 420 watts and you can get away with just 400 watts. With a 580 AH battery is 696 watts, just call it 700 watts. As for a MPPT charge controller is easy, you would need either at least a 35 or 60 amp MPPT Charge controller respectively. RV's are easy huh?

      Now allow me to make you feel better. If you install an Electronic Battery Isolator which is a must have item in an RV, and drive a couple of hours every day, you will not need any Solar Anything. Your Engine Alternator can generate more energy wiht two hours drive time than your roof full of panels can generate in 2 days. An Isolator will also reduce Generator run time requirements. A good Isolator cost less than $75 USD. So if you drive everyday for a couple or hours and still want solar, then cut back to a 200 watt panel with a 15-amp MPPT Charge Controller for show and tell.
      Last edited by Sunking; 05-16-2018, 01:57 PM.
      MSEE, PE

      Comment


      • #4
        Hi guys!
        Thank you for your answers!
        I did understand the post from you sunking, and I understand that most of the forum is about large installations in fixed locations. But you are the ones with the knowledge, and the best advices.
        It's true what you both say about an isolator, and I've given a good thought. But I didn't express myself quit correctly. my Rv is a caravan... So I Don't have it every time engaged with the truck. Although could do... and that's why give it the time to think, of doing the isolator installation, but truely I'm not sure yet how to... cables should run long from the alternator... Not sure the size.
        So, as being a caravan, most of the time we go to places where the truck isn't started for a few days. It's more easy to have a fixed solar system to keep those watts we need for sure. Obviously I've tried to oversize our requirements... it's probably we use less, so if I did the math correctly, I'd start looking at the components, that will ask to you guys too. Here isn't too much information and thats why I joined the forum. People who has the knowledge, sells what they want... nobody tells you how to plan your solar system, and I imagine most of the people does wrong shopping. All about solar systems is expensive and I've learned reading the forum, that this is not as green as everyone says.
        The truth is probably I'll get to 350 amp battery bank as max capacity. Do you think I could do with a smaller system?
        Another question... If I go with the 300... 350 watts panels, is it better to have 3 or 4 100 watts panels than 1 of 350 watts?
        Sunking, for what I read, the system should be 12v, as I'll be far below 600w system. The math gave me the same numbers ass you... 341 for 3 days. 583 for 5 days... Excesive in wheight for my caravan.
        How do I plan the battery bank? using 6v batteries? I've read that is better using smaller volt batteries, but the only deep cycle we get here are 12v and 6v. There's nothing in smaller volts.
        Again, thank you guys for your answers. I'm learning a loooot.

        Maxi

        Comment


        • #5
          Originally posted by maxgarde View Post
          Hi guys!
          Thank you for your answers!
          I did understand the post from you sunking, and I understand that most of the forum is about large installations in fixed locations. But you are the ones with the knowledge, and the best advices.
          It's true what you both say about an isolator, and I've given a good thought. But I didn't express myself quit correctly. my Rv is a caravan... So I Don't have it every time engaged with the truck. Although could do... and that's why give it the time to think, of doing the isolator installation, but truely I'm not sure yet how to... cables should run long from the alternator... Not sure the size.
          All you have to do is ask. Wire size from Alternator is a bit different than say from the controller or battery to loads because we are not to concerned about voltage drop and can use some resistance to limit charge current. May sound illogical at first until you can see the whole picture. Truck and RV alternators to be high current output. More then most batteries except the starting battery can handle. This makes things pretty simple. You base the size of the wire to the minimum wire requirement set by the fuse or breaker. Using Marine wire with say a 50 to 70 Amp fuse all you need is 6 AWG. Between the fuse and wire resistance will limit the current. trust me makes no difference if you a park for a week or overnight, a Isolator is mandatory equipment.

          MSEE, PE

          Comment


          • #6
            Originally posted by maxgarde View Post
            The truth is probably I'll get to 350 amp battery bank as max capacity. Do you think I could do with a smaller system?
            Battery? If you have determined your daily usage correctly, 300 to 350 AH battery is a good fit.

            Originally posted by maxgarde View Post
            Another question... If I go with the 300... 350 watts panels, is it better to have 3 or 4 100 watts panels than 1 of 350 watts?
            I would use higher voltage panels because they are generally less expensive than lower voltage battery panels. However what ever it takes to limit to no more than two panels, at 350 watts can be done with 2 x 175 volt 12 volt battery panel wired in series. On a RV roof dimensions limits your physical size choices.


            Originally posted by maxgarde View Post
            Sunking, for what I read, the system should be 12v, as I'll be far below 600w system. The math gave me the same numbers ass you... 341 for 3 days. 583 for 5 days... Excesive in wheight for my caravan.
            How do I plan the battery bank? using 6v batteries? I've read that is better using smaller volt batteries, but the only deep cycle we get here are 12v and 6v. There's nothing in smaller volts.
            RV's unless you just have excessive power needs are 12 volts for a number of reasons like gizmos made for RV's. Technically you can take 12 volts up to 1000 watts with a 80 Amp MPPT controller. The same controller can go to 2000 watts @ 24 volts and 4000 watts @ 48 volt battery. In my book limit 12 volt systems to 500 to 600 watts for a number of reasons I will not go into now.

            Relax on the battery issue. You are right about lower voltage batteries. Again it a weight and capacity issue. Find a 12 volt 200 AH battery if you can, and you will need a lift to pick it up. But here is what you need to know, use 2 x 6-Volt batteries wired in series of the right capacity. 6-Volt batteries range from 180 to 900 AH and everything in between. So it is just a matter buying the correct capacity. What manufactures are available to you down there? I can help you select.

            12 volts range from 5 AH to about 150 AH
            8 volts 160 to 400
            6 volts from 200 to 900 AH
            4 volts 500 to 1500
            2 volts 1000 AH to 8000 AH
            Last edited by Sunking; 05-17-2018, 04:01 PM.
            MSEE, PE

            Comment


            • #7
              Hi again guys,

              Thank you for your explanations. I understand some more of all of this.
              Most of what I can buy in my country you may see it in ths page www.mercadolibre.com.ar using words such as "panel solar" for the panels, "bateria ciclo profundo 6v" for the batteries. "regulador mppt".
              Money is an issue. Exchange currency dolar pesos is 1-25 actually.
              I saw some 90 watts panels and I thought of using 4 of them connected in series for a high voltage output to the regulator... Is this ok?
              Also I was thinking of building an external box to put batteries and regulator. Something well insulated from climate and caravan chasis.
              What do you think?
              Last edited by maxgarde; Yesterday, 10:36 AM.

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