Confused walking into an incomplete off grid setup

akchaffeeakchaffee Registered Users Posts: 4

I recently purchased an off grid place and it has some existing solar equipment setup. From my limited knowledge, It is a 24V system with 12-2VDC batteries connected to an Outback FM 80 charge controller. There are 4 Sun-A-205-fa3 panels with the wiring all just electrical taped together (Really want to get this fixed). I have 4 additional, matching, panels that need to be installed. I want to add these four panels and get the entire array setup correctly but I am having difficulty in finding the knowledge to do so.

With a 1640W setup of 8 panels what is the best way to wire these? Two strings of series put in parallel?
What combiner box should I be using and what gauge wire to the house? (150 foot from array to charge controller)

The system is also using a VFXR3024E inverter. I have a second one in a box that I am guessing the intent was to stack with the installed at some point.

Does this system seem balanced and where should I focus on improvements?


  • BB.BB. Super Moderators, Administrators Posts: 32,593 admin
    Welcome to the forum akchaffee,
    Can you tell us a bit more?
    • Amp*Hour capacity (20 hour rate) of battery bank (Lead Acid, AGM, or what)?
    • Can you tell us more about your panels--Vmp (voltage maximum power) and Imp (current maximum power) ratings?
    • Rough location of system (nearest major city--Figure out hours of sun per day by season and min/max temperatures)?
    • Daily use? Weekends and sunny weather?
    • Loads... Watt*Hours per day or Amp*Hours @ 24 VDC per day (you may not know)...
    • If you don't know your energy usage, what appliances/LED lights/computer/water pump/washing machine/etc.?
    • Especially for Lead Acid batteries--Is the battery bank charged/kept charged (genset once a month, at least some solar, etc.)? Lead Acid batteries do not like being stored at less than ~75% state of charge.
    I suggest we go through this setup a step at time... Makes things a bit easier to follow.
    Also, lead acid batteries--You want to measure their resting voltage and specific gravity (if flooded cell)--They need to be checked (at least, typically, once per month) to make sure they are "happy"... If not happy, they can pretty quickly sulfate and die--And leave you with scrap lead.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PhotowhitPhotowhit Solar Expert Posts: 5,803 ✭✭✭✭✭
    I think I can help on the panels.
    The "Sun A-205-fa3" solar panels are very likely some of the remaining stock of Evergreen Solar Panels as they went out of business, Sun Electronics bought up the remaining stock in several areas. So labeled some not, some 2nds. I bought my current array back in 2012 during this huge sale (something like 100 containers from Europe alone).

    I think the label would indicate 205 watts each, These are lower voltage than you might thing, with specs of roughly ;
    • Imp 11.27A
    • Vmp 18.2V
    • Isc 11.93A
    • Voc 22.7V

    Here is a link to the PDF of Evergreen ES-A-205-fa3 panels;

    Evergreen ES-A-205-fa3 (205W) Solar Panel (

    To figure out how 'balanced' a system you have we would need to know the amp hours of the 2 volt batteries, knowing the model number of the 2v cells would be good to establish if they are designed for deep cycle use or float use. They should be charging with what ever you have up and running, and kept topped off with distilled water(water with minerals will ruin the batteries!)

    Also where in the world are you? the available amount of sun is much greater in the southwest than the north west of ohio river valley.

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former, 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • akchaffeeakchaffee Registered Users Posts: 4

    Hello! There are quite a few aspects of this I had not considered.

    ·        Amp*Hour capacity: They are Lead Acid. I also believe they have been well maintained. Checked monthly with distilled water. The one thing I don’t currently have is the specifications on the batteries. I will be back at the place in a few weeks and can get the information then. All I recall at the moment is they were 2volt, pretty large and redish in color. My guess is something similar to the Trojan L16RE-2V 1110AH as they were the same color/size/yellow caps.

    ·        Photowhit is correct on the panel ratings.

    Pmp 205W, Imp 11.27A, Vmp 18.2V, Isc 11.93A, Voc 22.7V

    ·         The closest town is Midway, BC. However, exact location is Southeast of there and 1600 feet on the USA side of the border in Washington.

    ·        Currently weekend or week duration use. My plan is to live there full time within a year.

    ·        Loads… working on actual numbers for this one. Appliances: water heater, stove/oven, refrigerator are all 100% propane. I will be installing a washer (electric)/dryer (gas). Heat is wood primary and gas backup. The water is currently gravity fed Spring but I will be installing a pump to lift the pressure at the house a bit. All lights are LED. It will be a family of 6 living in a 2800sqft house with basic electronic usage.

    ·        Of the few times I have been out there the batteries have achieved a full charge each day. I have a 6500w generator sitting out back for when the need arises.

  • wellbuiltwellbuilt Solar Expert Posts: 763 ✭✭✭✭
    Sound like a good start
     I would add as much solar power as your fm 80 controller can handle . 
      I would think you could have a total of 12 panels @24volts and be ok on your fm 80 . 
      My system is 3600 watts 48 volts but I have 4500 watts of solar on my fm80 
     I’m over paneled by 500 or so watts . 
     I have a second fm80 and 4500 watts to be installed 
      My place is 2800 sf +- I’m in ny in a very snowy location and come December it is very dark with little power from the sun . 
      The good news is by the second or 3rd week in January I start getting some decent sunny days. 
     My solar panels where covered with snow starting at Christmas for 10 weeks so my generator ran every morning for most of the winter . 
     I charge my battery’s with a 2800 watt Honda it maxes out my out back inverter charger and leaves some power to pass thru to loads  , it  doesn't burn much fuel 5 gallons a week . 
     I’m running my regular loads but no Landry and I unplug the freezer  I do run the Toaster and microwave a little.
      It seams to me that the more I draw down my battery’s the longer they take to charge.
       I  charge for a hour or 2 every day and reach full charge every 3/4 days . 
     I try not to go below 70% ever because if I do it takes a long time to bring the charge back up. Good luck with you system 
    Out back  flex power one  with out back 3648 inverter fm80 charge controler  flex net  mate 16 gc215 battery’s 4425 Watts solar .
  • akchaffeeakchaffee Registered Users Posts: 4
    So with the panels I have available I have a total of 1640W. If I run two strings to a combiner box that puts me at 72.8V and 22.54A? Is this a good setup for the FM80 charge controller? What combiner box (fuses/breakers) and cable size should I be using? 150 feet between combiner and controller.
  • wellbuiltwellbuilt Solar Expert Posts: 763 ✭✭✭✭
    I used this combiner box for 5 strings of 3 panels 
     it can be converted for rapped Shut down if necessary . 
     They have smaller boxes available I think I use 15 amp breakers that I bought from AWS if you call the store the guys are very knowledgeable . 
     I used a bulk spool of  PV under ground uv 10g in red and black my runs are 70’ +-  I bought a Quality crimper and added my own ends . 
     I ran 5’ from the box to my fm80 . 
     I would plan on adding a third string , your fm 80 will take a in put of 2000watts of solar @24 volts but I think you would be ok over paneling to 2400watts . ( Im no expert ) 
      With 2 strings you may not need a combiner box ? But it makes it easy to turn off a string if necessary . 
      I don’t think you can have to much solar power with full time off grid .
      This time of year I’m making lots of power I’m in absorb at 1100 and floating by 100  
      Things are not that easy come December you will need to run your generator every day or 2 
     1600 watts of solar will bring in 2400 watts a day or so . 
     2400 watts of solar could make 3600watts a day 
           I’m sure photowhit and BB will chime in with more information 

    Out back  flex power one  with out back 3648 inverter fm80 charge controler  flex net  mate 16 gc215 battery’s 4425 Watts solar .
  • BB.BB. Super Moderators, Administrators Posts: 32,593 admin
    edited May 2021 #8
    You kind of are in between a rock and a hard place... The Evergreen panels are no longer available (that I am aware of--I am not in the business).
    Generally, for a long run from array to the charge controller, you would want Vmp-array to be relatively high, and the array current relatively low--That allows you to use smaller diameter/AWG wiring and save money.

    Using a simple voltage drop calculator, and 3% to 1% typical wiring drop (more than 3%, you are wasting power, below 1% you are wasting money). 72.8V and 22.54A @ 150 Feet. Playing with the numbers we get:

    With 4 AWG:
    voltage drop: 2.38
    Voltage drop percentage: 3.27%
    Voltage at the end: 70.42

    Depending on how cold it gets (Vmp and Voc rise as the temperature goes down towards freezing and below)--Say 6 panels in series * 18.2 volts per panel = 109.2 volts @ 11.27 amps:

    With 8 AWG:
    Voltage drop: 2.99
    Voltage drop percentage: 2.74%
    Voltage at the end: 106.21

    Of course, this is only with 6 panels total--Two panels left over (no more panels to purchase for a second controller)... Vmp-array @ 109.2 volts may be too high for your region (22F = 150 Voc-cold--right at the limit for the FM 80). Using Midnite's string sizing tool (easy website to use).
    PV Array
    Rated PV Array Power:1230Watts
    Anticipated Array Power @ 104F:1147Watts
    Rated PV Array Current:11.27Amps
    Battery Charging Current @ 28.8 V:42.7Amps
    VMP (Maximum Power Point Voltage) :109.2Volts
    VOC (Open Circuit Voltage):136.2Volts
    VMP @ 22 F°:124.2Volts
    VOC @ 22 F°:150Volts
    Classic, Classic SL & Classic Lite Charge Controller Selection
    Max Operating Voltage150200250
    Max Non operating VOC (HyperVOC) @ 24V Nominal Battery Voltage174224274
    Maximum Number Of Modules In Series6810
    Max Number Of Modules In Series (Using HyperVOC)6810
    Max Allowable Output Current Per Classic
    Based On This Current Configuration
    Max Allowable Wattage Per Classic
    Based On This Current Configuration
    Present PV Array Wattage Of This Configuration123012301230

    If you could get more panels to add to your array--You could do 5 or 6 in series, and 2 parallel strings.. The suggested maximum array for a FM 80 controller @ 24 volt battery bank is 2,000 Watts from the specs... Suggest over paneling upwards of:
    • 80 amps * 29 volts charging * 1/0.77 panel+controller derarings = 33 Watt array suggested over panel maximum
    • 80 amps * 29 volts charging * 1/0.77 panel+controller deratings = 3,013 Watt array suggested over panel maximum
    • 3,013 Watt max / 205 Watt panels = 14.7 or 7s x 2p max (too high of Vmp/Voc array for your location)
    • 2,000 Watt array / 205 Watt panels = 9.8 ~ 5s x 2p max (good fit, but 2 more panels than you currently have)
    Your Voltage Drop calculation for 5*18.2=91Vmp and 25.54 Amps for 2p strings @ 150 feet:

    For 4 AWG:
    Voltage drop: 2.38
    Voltage drop percentage: 2.62%
    Voltage at the end: 88.62

    If you had a 48 volt battery bank, you could install a 2x larger array for the FM80 (save money--Don't need second FM80 for more panels). However, your battery bank may need to change--And need a new/different AC inverter for 48 VDC bus.

    And you did not tell us what type of batteries you have, and their Voltage/AH rating and ?series by ?parallel connections.

    Like to have loads drive battery bank type/size... And the loads+battery bank size+hours of sun per day/etc. => solar array. So you end up with a "balanced" system design.


    PS: Fixed typo above from 33 to 3,013 Watt array over paneling.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • wellbuiltwellbuilt Solar Expert Posts: 763 ✭✭✭✭
    Nice bill , I really like reading your explanations ! 
    Out back  flex power one  with out back 3648 inverter fm80 charge controler  flex net  mate 16 gc215 battery’s 4425 Watts solar .
  • BB.BB. Super Moderators, Administrators Posts: 32,593 admin
    edited May 2021 #10
    Thank you Wellbuilt... I am unsure how to write/answer questions like this. Still don't have all the information (battery bank) or really understand the power needs... So I get conflicted on how to best answer.

    I did forget that the battery bank is (something like) 2 volts @ 1,110 AH... So I will do some more math on the basic system sizing:

    What seems to be pretty  optimal for a lead acid battery system... 2 days of storage (2 days of "no sun") and 50% max planned discharge (longer battery life).
    • 24 volts * 1,110 AH * 0.85 AC inverter eff * 1/2 days storage * 0.50 max discharge = 5,661 Watt*Hours per day
    That is a "reasonable" start for a very efficient home (I suggest a minimum of 3,300 WH per day for Energy Star rated refrigerator/freezer, LED lights, some water pumping, TV, laptop computer, washing machine on sunny day).

    Two calculations for solar array sizing... One based on size of battery bank (larger bank, more panels). 5% can work for summer/weekend/sunny weather usage... 10%-13% typical minimum for full time off grid (9+ months a year):
    • 1,110 AH * 29.0 volts charging * 1/0.77 panel+solar derating * 0.05 rate of charge = 2,090 Watt array minimum
    • 1,110 AH * 29.0 volts charging * 1/0.77 panel+solar derating * 0.10 rate of charge = 4,181 Watt array nominal
    • 1,110 AH * 29.0 volts charging * 1/0.77 panel+solar derating * 0.13 rate of charge = 5,435 Watt array "typical" cost effective maximum
    And sizing for daily energy usage vs hours of sun per day (your location, seasonal power needs summer/winter/etc.).

    PVWatts: Monthly PV Performance Data
    Requested Location: midway bc canada
    Location: Lat, Lon: 49.01, -118.78
    Lat (deg N): 49.01
    Long (deg W): 118.78
    Elev (m): 801.8400269
    Module Type: Standard
    Array Type: Fixed (open rack)
    Array Tilt (deg): 49
    Array Azimuth (deg): 180
    Month Solar Radiation (kWh/m^2/day)
    1 2.3 (i.e., January 2.3 hours of sun per day)
    2 3.5
    3 4.5
    4 5.5
    5 5.3
    6 5.3
    7 6.4
    8 6.3
    9 5.7
    10 4.0
    11 2.1
    12 2.180

    Say you toss the bottom 3 months (poor sun--Use genset, less power in winter?):
    5,661 WH per day * 1/0.52 off grid system AC eff * 1/3.5 hours of sun "February break even" = 3,110 Watt array Feb "break even"

    Based on the present battery bank size and load guesses... For a full time off grid system, I would be suggesting a 4,181 Watt through 5,435 Watt array--And at least 3,110 Watt array minimum (assuming 9 months a year, not too much genset usage).

    Or, if you keep the battery bank size and voltage, that would be at least 2x FM80's for ~4,000 to ~6,600 Watt array capacity (2x FM80).

    Note: I have a typo in the earlier post--I will correct but the right answer should have been:
    • 80 amps * 29 volts charging * 1/0.77 panel+controller derarings = 33 Watt array suggested over panel maximum
    • 80 amps * 29 volts charging * 1/0.77 panel+controller deratings = 3,013 Watt array suggested over panel maximum
    I keep hitting the touch pad and moving my cursor in the middle of typing and blot out other text.

    Again, none of this is written in stone. I am a big believer in balanced system design. And keeping the battery bank "happy"--Which usually means a much larger than "default" array--Especially in areas with poor solar / poor winter solar. More panels is typically cheaper than more generator fuel, runtime, and maintenance...

    But the system has to meet your needs... Both in energy supplied and co$t.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • akchaffeeakchaffee Registered Users Posts: 4
    Thank you all for the input and Bill for the detailed education. This is going to take me some time to digest/review/and make some decisions.
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