Waving the little white flag

monicajmonicaj Posts: 2Registered Users
I've spent days and days trying to understand this stuff and apparently I've over estimated my mental capacity.  Bout the time I think I've almost got it, my brain pukes and retains nothing.

I bought 4 of these solar panels (for $1,000 + 200 shipping = good deal huh?  I think it's 72 cells?), and I'm trying to put 2 of them over on a friend's property and set them up for him.  

My first question:  Am I right that these are 24v?

Second question:  Should I connect them in series for 48v?  I keep reading that increasing volts instead of amps will save money by using smaller wire.  The wire they mean is the one going from the panels to the charge controller?  (I'm picturing the panels being maybe 40 feet from the camper and the charge controller is in the camper.)

More questions... 

Am I right that he should get an MPPT controller and use a 12v battery bank because he's going to be running 12v stuff (fans, lights, radio, maybe fridge...)?  and this question leads to the next...

5th question:  IF he should use a 24v battery bank,  then does he have to get something to change the volt from 24v down to 12v for his 12v stuff, and is it an expensive piece?

For some reason, he's wanting to have 2 separate systems... in case one goes down.  But is that necessary?  Because wouldn't the charge controller be the only thing that may fail, and couldn't that be protected with an inline fuse or breaker or whatever it's called to protect the controller?  So if he's really worried about that, couldn't he just make sure to have a second controller on hand?

IF it's a better idea to have 2 systems, then he could put each panel on a charge controller, and both charge controllers can charge one battery bank right?

And specifically, which charge controller/s should I get?  What specs should I be looking for?

A question about monitoring things... I saw something that can tell you exactly how much the panels are producing and how much juice is in the battery and a few other things... I think charge controllers can give you that info?  A cc that gives you all this info would probably be a lot more expensive so should he just spend a little less on the controller and get the gadget that monitors these things?   But if he has 2 systems, is there even a way to connect up that gadget so that it can read both charge controllers?  Or am I completely off base with how that works.

Lastly, how many amps should he have for his battery bank?

About inverters:  He does have an inverter but I think it's way to big.  He said it's 3,000 watts, but I haven't seen it yet or looked up the specs on it, so maybe it's not even an off grid inverter, but a grid tie one, but either way, wouldn't he do just fine with a smaller inverter, 1,000 watts maybe, and doesn't the big one use more juice to run?

If you can please help me get clear on this, my beat up brain thanks you profusely.

Comments

  • PhotowhitPhotowhit Posts: 4,448Solar Expert ✭✭✭✭

    They are 24v (nominal) panels, this is irrelevant if he will have a 12 volt system, Why?

    For a 12 volt system he will have to have a MPPT type charge controller. The charge controller will create the proper charging voltage for a 12 volt system.

    Charge controllers are very reliable, but they do fail. I'd rather have a spare than have double the money in batteries, charge controller and array(panels)

    Inverter sounds too large to me!

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • bill von novakbill von novak Posts: 719Solar Expert ✭✭✭✭
    monicaj said:
    My first question:  Am I right that these are 24v?
    Yes, they can be used for a 24V system.

    >Should I connect them in series for 48v?  I keep reading that increasing volts instead of amps will save money by using smaller
    >wire.  The wire they mean is the one going from the panels to the charge controller?  (I'm picturing the panels being maybe 40 feet
    >from the camper and the charge controller is in the camper.)

    Doesn't matter too much at these levels.  40 feet of wire isn't going to kill you on cost if you have to go up a few gauges.

    >Am I right that he should get an MPPT controller and use a 12v battery bank because he's going to be running 12v stuff
    >(fans, lights, radio, maybe fridge...)?  and this question leads to the next...

    If his loads are low (below about 500 watts, less than a few kwhr a day) AND he is going to use 12V appliances then 12 volts can work.  As you go higher in power higher battery voltages begin to make sense.

    > IF he should use a 24v battery bank,  then does he have to get something to change the volt from 24v down to 12v for his 12v stuff,
    >and is it an expensive piece?

    Not too expensive.  Depends on power level.  You can get a 200 watt converter for $20-$50.

    >For some reason, he's wanting to have 2 separate systems... in case one goes down.  But is that necessary?

    No.

    >And specifically, which charge controller/s should I get?  What specs should I be looking for?

    Look for a good name (Morningstar, Outback, Midnite.)  The Kid is a good small MPPT.  Look for input voltage range, current rating and ability to do multistage charging.

    >A question about monitoring things... I saw something that can tell you exactly how much the panels are producing and how much juice
    >is in the battery and a few other things... I think charge controllers can give you that info?

    The Kid (for example) will give you power into the battery.  To monitor battery state (i.e. "you have 40% left") you need a separate monitor like the Trimetric.

    >Lastly, how many amps should he have for his battery bank?

    Rules of thumb:

    Plan for no more than 20% discharge per day.  That gives you a few days of autonomy (no sun) without going below 50%, which is bad for batteries.

    Plan to charge at a rate of between C/5 and C/15. (C is capacity, so for a 100ah battery, C/10 would be 10 amps.)  In other words, if you have 32 amps of charge power, then you could handle roughly between 160 and 480 amp-hours of battery (at 12V.)  The battery manufacturer may have different numbers.

    >He does have an inverter but I think it's way to big.  He said it's 3,000 watts, but I haven't seen it yet or looked up the specs on it, so
    >maybe it's not even an off grid inverter, but a grid tie one, but either way, wouldn't he do just fine with a smaller inverter, 1,000 watts
    >maybe, and doesn't the big one use more juice to run?

    I am almost certain it's one of those Chinese "3000 watt" inverters that doesn't work.  (Just via simple math, 3000 watts at 12 volts would require 250 amps - far beyond the capacity of the terminals they use.  He would be much better off with a cheaper/better converter.





  • BB.BB. Posts: 27,214Super Moderators admin
    monicaj said:
    I bought 4 of these solar panels (for $1,000 + 200 shipping = good deal huh?  I think it's 72 cells?), and I'm trying to put 2 of them over on a friend's property and set them up for him.  
    • Sounds like a good price (I am not in the business).
    My first question:  Am I right that these are 24v?
    • I would prefer to call them Vmp~35.2 Volt panels. What you use them for (and behind what type of solar charge controller) is what matters. But, yes, this panels can (singly) charge a 24 volt Lead Acid Battery bank.
    Second question:  Should I connect them in series for 48v?  I keep reading that increasing volts instead of amps will save money by using smaller wire.  The wire they mean is the one going from the panels to the charge controller?  (I'm picturing the panels being maybe 40 feet from the camper and the charge controller is in the camper.)
    • What is your Battery Bank Voltage and what Brand/Model of solar charge controller (MPPT or PWM--Needs to be MPPT). Higher Vmp-array will let you use smaller AWG (light weight) cord. However, it will make the MPPT charge controller run a little less efficiently (controller will run a bit hotter)
    More questions... 

    Am I right that he should get an MPPT controller and use a 12v battery bank because he's going to be running 12v stuff (fans, lights, radio, maybe fridge...)?  and this question leads to the next...
    • Fridge can be a big load (relative to all other loads)--It will probably define if you use a 12 or 24 volt battery bank (plus fans+lights+etc. can consume a lot of energy too). For RVs, there are a wide variety in refrigerator types/power usage. For example a Gas Absorption type (propane) fridge on 12 VDC or 120 VAC is very energy wasteful.
    • really need to know how much power/energy you will use. For off grid and RV power from solar, you really need to conserve energy (LED lighting, small/energy efficient fans, turn stuff off when not used, etc.).
    5th question:  IF he should use a 24v battery bank,  then does he have to get something to change the volt from 24v down to 12v for his 12v stuff, and is it an expensive piece?
    • Yes, there are DC to DC converters... But if you can use a small/efficient AC inverter--Would that work for you?
    For some reason, he's wanting to have 2 separate systems... in case one goes down.  But is that necessary?  Because wouldn't the charge controller be the only thing that may fail, and couldn't that be protected with an inline fuse or breaker or whatever it's called to protect the controller?  So if he's really worried about that, couldn't he just make sure to have a second controller on hand?
    • Yes, things fail. Having backup parts is sometimes needed. Also, if full time off grid, will there be a genset available--That can be your backup energy source.
    • If loss of power means loss of life (or loss of work)--Then the person should use the rule of three for backup. I.e., assume that the primary "thing" failed, and the secondary/backup failed too. The third backup is still available.
    IF it's a better idea to have 2 systems, then he could put each panel on a charge controller, and both charge controllers can charge one battery bank right?
    • It depends. Yes, you can have parallel power systems and if one fails, the second would be available. However, you now have 2x more parts and 2x more likely something will fail, and possibly it will be harder to debug/fix (more complexity, more confusion, more ways for things to fail).
    • In general, for most people, keep it simple. And have some backup parts on the shelf, and one (or two) backup gensets.
    And specifically, which charge controller/s should I get?  What specs should I be looking for?
    • Lets talk about sizing your system first. There is no "one ideal" device that works in any size system.
    A question about monitoring things... I saw something that can tell you exactly how much the panels are producing and how much juice is in the battery and a few other things... I think charge controllers can give you that info?  A cc that gives you all this info would probably be a lot more expensive so should he just spend a little less on the controller and get the gadget that monitors these things?   But if he has 2 systems, is there even a way to connect up that gadget so that it can read both charge controllers?  Or am I completely off base with how that works.
    • There are many ways to monitor your system (locally, remote over the Internet, via charge controller/inverters themselves, separate monitoring devices, etc.). Cost and complexity are usually important. Are you somebody that wants the system "to work"--Or do you want hourly data to graph/put into a spread sheet? Do you want the system to email you if there are problems?
    Lastly, how many amps should he have for his battery bank?
    • Need to know your loads first (suggest you choose energy efficient devices--cheaper than building a larger solar system).
    About inverters:  He does have an inverter but I think it's way to big.  He said it's 3,000 watts, but I haven't seen it yet or looked up the specs on it, so maybe it's not even an off grid inverter, but a grid tie one, but either way, wouldn't he do just fine with a smaller inverter, 1,000 watts maybe, and doesn't the big one use more juice to run?
    • Yes--Especially for RV's, you are better off running small loads and small AC inverters. In some cases, you may want a second/large AC inverter to run a power tool for a few minutes, etc... I would humbly suggest a 300 Watt 12 VDC TSW Inverter should keep most RV folks happy (propane fridge, not large motor homes) and 2 to 4x 6 volt @ 220 AH "Golf Cart" batteries wired to a 12 volt @ 220 or 440 AH battery bank.
    If you can please help me get clear on this, my beat up brain thanks you profusely.
    • I also suggest that your next posts focus on one subject at a time (I suggest picking your loads and measuring/estimating your energy usage). That will drive much of the rest of the system design.
    • If you do this a step at a time, you will better understand the entire solar power system at the end of the process. It is difficult to "understand everything" at one time--Like drinking from a fire hose.
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bill von novakbill von novak Posts: 719Solar Expert ✭✭✭✭
    monicaj said:
    wouldn't he do just fine with a smaller inverter, 1,000 watts maybe, and doesn't the big one use more juice to run?
    Some gear recommendations:

    Midnite Kid charge controller $300
    Morningstar Suresine 300 watt inverter $220 (he will say "but I can get a 3000 watt inverter for $100 on Amazon!" Do not listen to him.)
    Samlex SA-600-112 600 watt inverter $300

    Also be sure to use appropriate fusing and disconnects.  The most critical place for a fuse/breaker is on the + terminal of the battery, either right at the terminal or as close as possible to the terminal after a SINGLE cable to the terminal.  Make no connections to the battery other than on the far side of the fuse/breaker and ensure that every single wire that goes to that far side of the fuse/breaker can handle the rated current of the fuse/breaker.  He will want to attach skinny wires to that terminal because they are cheap.  Again, do not listen to him.  Every wire in the system has to have a fuse or breaker between it and the battery that will blow before the current in that wire can melt it or cause a fire.
  • monicajmonicaj Posts: 2Registered Users
    edited May 2016 #6
    Thank you Bill and BB.  You don't know how much I appreciate you taking the time to respond. 
  • DanKegelDanKegel Posts: 15Registered Users ✭✭
    Why do people say they are 24V panels?  The datasheet says their output voltage at the maximum power point is more like 31 volts.
  • littleharbor2littleharbor2 Posts: 814Solar Expert ✭✭✭✭
     Vmp is 35.2 volts. In the off grid world this is considered a 24 volt (nominal) module.  In the early days when PWM controllers were considered state of the art and there weren't 60 cell grid tie modules  you needed to have something in the 35-36 volt range to charge a 24 volt battery bank.  12 volt modules were much more common than they are today and were simply series wired to achieve your 24 volt nominal voltage range or use 72 cell 24 volt panels.
      Nowadays MPPT controllers are the "state of the art", for lack of a better term. With these controllers you intentionally input higher than battery bank voltage and the controller does the stepdown while harvesting maximum power from your array.
     A 31 volt, 60 cell module you are referring to doesn't put out high enough voltage to properly charge and equalize a 24 volt battery bank. They are considered a 20 volt nominal module. BUT with an MPPT controller you can series wire at least two 60 cell, 31 volt modules, increasing the voltage high enough for the MPPT controller to do it's magic.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

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