I have 8) 100w panels, a mmp solar 3kw 48v to110v inverter and a 6kwh 48v battery.

My panels i want in the yard about 150ft out.   I have a shed at the mid way point that i could put the inverter and battery into. .

The inverter is rated for 60v to 115v input from the panels.

I hear the inverter should be close the the house? but the i hear its good to have the highest voltage on the longest run,, whether ac or dc. 

Should i split the difference and use the shed as a battery/inverter box.  (kinda like the idea)

Trying to decide on 4s2p on panels ( short run) or  series all the panels for 96v and put inverter and battery close to the house? 

First poast.  Thanks


  • joelthewelderjoelthewelder Registered Users Posts: 12 ✭✭
    I can also move the shed out closer to the panels or move the shed closer to the house...

    Any advice appreciated. 
  • joelthewelderjoelthewelder Registered Users Posts: 12 ✭✭
    So i realize now my voc 22.6 is too much for all 8 panels.  My inverter says 145v max for voc.. .i will stick to 2 strings of 4.   
  • BB.BB. Super Moderators, Administrators Posts: 30,634 admin
    Welcome to the forum Joel,

    The short answers are that you want the highest voltage to be the longest distance--That lets you use the smallest diameter cable (less copper, less costly cable). From a copper point of view, 60 VDC needs something like 6x heavier copper cable vs 120 VAC.

    Sort of, the "best" run to make longer is the solar panel wiring. In general, there is no surge current, and placing the "battery shed" next to the house means it is easier to go out for a quick inspection of the hardware, batteries, etc.

    While rules of thumbs gives us a nice starting point. Details matter. And, for some applications, more expensive charge controllers that support higher Vmp-array voltages can be nicer for those long(er) distance runs.

    The other thing I suggest doing is looking at the hardware/setup you already have, and use some math and rules of thumbs to get the system "balanced" (matching battery bank to loads, solar array to battery bank, loads, hours of sun per day, inverter size, etc.).

    For example, your present 3 kwatt inverter would drain a 6 kwatt*hour battery bank in less than 2 hours. For a normal off grid system, that inverter is probably too big for that battery bank for a "useful" off grid home power system.

    However, perhaps you have special needs... "Welder"? Low duty cycle, high power for short periods of time. Starting a well pump. You have AGM or other high current batteries that "surge better" than flooded cell lead acid batteries.

    Also, there are usually some hardware specifications/limitations. For example, a 48 volt battery bank needs to be charged at ~60 Volts. And a Vmp~60 volt solar array, on hot days, the output (Vmp--Voltage maximum power) can drop to 50 VDC or so--And limits the charging current to the battery bank. In general, a 48 volt battery bank should have a Vmp-array (standard test conditions) of ~72 volts minimum. And Voc-array-cold for a Vmp=72v array is very close to the 115 VDC max input (Vpanel max for charge controller) for your controller(?).

    You can give us some more information (hardware brands/models, battery bank chemistry, nearest major city for hours of sun, what your loads per day are, etc.) and we can try to you design and build out a "balanced system".

    In any case, the answers to your questions about cables. First you need to design the wiring to carry the current. A simple NEC chart is a good starting point:

    For an 800 Watt array (100 watt panels), Pmp~100w; Vmp=100w/17.5Vmp=5.71AImp (panel specs). A 4x * 2p array (starting point):
    • 4 * 17.5 Vmp = 70 VDC Vmp-array (std)
    • 2 * 5.71 Imp = 11.42 Imp-array (std)
    Using the NEC, 14 AWG is heavy enough for current (using the correct wire--Outdoor/UV rated if in sun, wet rated if in conduit, etc.).

    Next, what is the voltage drop for 150 Ft (one way run for this voltage drop calculator), 70 VDC, 11.42 Amps:


    14 AWG:

    Voltage drop: 8.65
    Voltage drop percentage: 12.36%
    Voltage at the end: 61.35

    Not good... Generally we look for ~1-3% voltage drop (fewer losses). Playing with the AWG number, find:

    4 AWG:
    Voltage drop: 0.85
    Voltage drop percentage: 1.22%
    Voltage at the end: 69.15

    Much more expensive cable....

    Now do the same thing for a 3,000 Watt @ 120 VAC inverter:
    • 3,000 Watts / 120 VAC = 25 Amps
    From the NEC,10 AWG is the minimum wire AWG. Drop calculator 120 VAC, 150 feet:
    10 AWG:
    Voltage drop: 7.49
    Voltage drop percentage: 6.24%
    Voltage at the end: 112.51

    Still too much drop... Try with different AWG:
    6 AWG:
    Voltage drop: 2.96
    Voltage drop percentage: 2.47%
    Voltage at the end: 117.04

    Somewhat better (note that 3kW inverter >> 800 Watt array; 120 VAC is only a bit higher than 70 VDC).

    So, in your case, because AC inverter >> Solar array --- There is not a "big" advantage of AC run over a DC run.

    HOWEVER, this depends on what you really need... Do you need 3 kWatt 120 VAC? Or less? Will you need a >> 800 Watt array (battery bank size, your daily loads, your hours of sun per day, etc.)...

    I highly suggest we start at the beginning (loads, location, your needs such as weekend/backup power, full time off grid or what). At this point, your AC inverter is huge vs the solar array wattage, and your battery bank is small for this size AC inverter).... For example, an off grid home/cabin with a 6 kWatt*Hour battery bank--I would be suggesting a ~600-800 Watt AC inverter, not a 3,000 Watt inverter (drawing huge amounts of current from a "smallish" battery bank).

    Your thoughts?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • joelthewelderjoelthewelder Registered Users Posts: 12 ✭✭
    Hey Bill, thanks for the help.  My battery is a lithium ion 120ah. I will be building 3 more batteries in parallel soon.  (24kwh Over the next year)  i will be adding more panels too over time.   This will be for nightime use for now.  Lights.and 400w pellet stove. 

    Still trying to get through your post.  Wow... ;)

  • joelthewelderjoelthewelder Registered Users Posts: 12 ✭✭
    Battery info. Its a 14s pack lg chem.  N2.1
  • joelthewelderjoelthewelder Registered Users Posts: 12 ✭✭
    Here is the specs on the right inverter, sorry about that. Mine is the 3048lvmk
  • joelthewelderjoelthewelder Registered Users Posts: 12 ✭✭
    To the rest of your questions,  i am in marysville washington.  The hot water tank is my biggest load,  but im not worried about that or the washer and dryer as of yet.    We have a fridge and freezer and a dishwasher.  

    No, im not going full time off grid anytime soon. 4 year plan perhaps.
  • joelthewelderjoelthewelder Registered Users Posts: 12 ✭✭
    I am pretty impressed with usa mpp solar as a seller..  I ordered the inverter 2 days ago and its here already with free shipping.  

    The feature that i like when i was researching it is that i can hook it to ac, charge the batteries, then run off the battery until its empty and it switches the load to ac power instantly.  

    I can also buy another one and parallel them in single phase up tp 9 units for 27kw max

    Or 9 units can work together to make 3 phase or split phase..  

    Thanks again bill for your examples and calculators.  They are very helpful.
  • joelthewelderjoelthewelder Registered Users Posts: 12 ✭✭
    I ordered parts to build a second lith battery so i will soon have 12kwh 240ah battery bank.  
  • BB.BB. Super Moderators, Administrators Posts: 30,634 admin
    Sounds intersting Joel.

    I would suggest you aim for 3,300 WH per day (3.3 kWH per day or ~100 kWH per month). That will support an Energy Star full size fridge, LED lightning, washing machine, solar friendly pump, laptop charging, cell charging... A pretty near normal "electrical life" with lots of conservation (using other sources of energy for heating, hot water, cooking, etc.).

    I can run the system design math for you, if you would like.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • joelthewelderjoelthewelder Registered Users Posts: 12 ✭✭
    You have been most helpful already Bill.  On the number of panels on a string question, from what i can tell based on the inverters pv 145v max and one of the calculators you sent. It looks like i could get 5 panels on a string and still be below the max volts .  (90v vpm and 113v voc)  ?  Is that too close for comfort?  

    Just trying to get more voltage for the long run..  i will try it with 4 and see if it works ok first.  Best to leave some safety room.

  • BB.BB. Super Moderators, Administrators Posts: 30,634 admin
    The problem is that solar panel specs are with the panels at 75F/25C ("flash" testing of panels--Just a few seconds of test light to confirm Voc/Vmp/Isc/Imp/etc.).

    I am guessing that it does get a bit below freezing there at least a few days a year... Vmp and Voc rise as temperatures fall--So you have to look at the Voc-array-cold value.

    In theory... Say 25F is your minimum temperature (-4C). Standard temp is 25C. Voc rise is -0.45% per C (typical)... So, your array should max out around:
    • 5 * 22.6 Volts Voc-std = 113 Volts
    • -4C min temp -25C std temp = -30C spread
    • -30C spread * -0.0045 ratio/V offset =  0.135 ratio increase
    • 113 Vmp-array-std * 1.135 increase = 128.3 Volts Voc-array-@25F
    So, it looks OK with 5 panels in series (assuming my minimum temp guess is right).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • joelthewelderjoelthewelder Registered Users Posts: 12 ✭✭
    edited June 13 #14
    Thanks Bill, the calculator was not specific to my inverter but thank you much for clearing it up for me.  

    One of the solar temp maps says the record low is 15f for marysville so your not far off.  

    Would the mppt in the inverter care either way as long as it does not go over 145vor under 70v.
  • BB.BB. Super Moderators, Administrators Posts: 30,634 admin
    edited June 14 #15
    You do not want Vmp-array to fall below ~72 volts (hot array, lower Vmp-array-hot). As you need almost 60 volts to charge the batteries, a bit for voltage drop, voltage drop across the charge controller (panels to batteries), etc...

    Otherwise, not really any issue if Vmp-array is in between. MPPT charge controllers tend to get a bit less efficient the higher the array voltage (by 1-2% typically) the higher the array voltage--But that is a small loss of energy (you could not really measure without laboratory equipment).

    Just keep the inverter/electronics in a well ventilated area (hot electronics die sooner vs cool electronics).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • joelthewelderjoelthewelder Registered Users Posts: 12 ✭✭
    Panels were delivered yesterday.

    Mppsolar also suggested to sticking to 4 panels per string.  

    Time to weld up a solar tracker stand..

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