12v DC vs 120v AC - 150 ft loop

thompsonbr87
thompsonbr87 Registered Users Posts: 3 ✭
in Solar Beginners Corner #1
I've got an off grid storage building, and I'm planning to use solar power for lighting and charging a few small electronics. I've been expecting to do a 12V DC system for simplicity sake, but I'm running into a delimma because of the distances required. I've got it down to about a 150' with a loaf of about 40 watts. I really would like to have another 20 watts in there though. Should I consider using a small power inverter and run this as 120v AC?
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #2
    Yes, since it's lighting, consider using the inverter as the on/off switch.
    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 backup...lol), 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.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    edited October 2018 #3
    You can use a voltage drop calculator to see what happens... 40 Watts:
    • 40 Watts / 12 volts = 3.33 amps
    150 feet (one way for this calculator) and say 1 volt maximum drop at 3.33 amps and playing with the AWG in the calculator:

    https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=3.277&voltage=12&phase=dc&noofconductor=1&distance=150&distanceunit=feet&amperes=3.33&x=66&y=20

    10 AWG cable...

    Now do 120 Volts @ 40 Watts and you get 0.333 amps and allow for 4 volt drop... Run the numbers:

    https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=133.9&voltage=120&phase=dc&noofconductor=1&distance=150&distanceunit=feet&amperes=.333&x=54&y=31

    26 AWG wire...

    It is just so much easier to send energy long distances at higher voltages.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #4
    Is the 150' largely between pv and battery, or between battery and loads?

    If from pv, you might consider higher dc string voltage to mppt controller. If between battery and loads, inverter likely makes more sense.

    For my low voltage lights etc (mostly 3-4w LEDs), I ran heavier wire drops to area junctions, and lighter wire to individual fixtures, which worked out well.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • thompsonbr87
    thompsonbr87 Registered Users Posts: 3 ✭
    #5
    PV to controller and battery will be pretty short - maybe 8' total. The 150' loop is battery to load.
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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    #6
    You can spend your money on thicker wire or DC-DC converters (cheap on ebay) or use 120VAC. The first is probably more expensive, but it's very reliable and has no standby or conversion losses.

    I am available for custom hardware/firmware development

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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #7
    Cost wise it might be nearly a wash, or maybe a bit cheaper for heavier wire vs a good inverter (eg Morningstar PSW 300w).

    One other consideration is whether loads can handle the voltage variation (assuming battery based), ~11-16vdc. Some might need a tighter range.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • thompsonbr87
    thompsonbr87 Registered Users Posts: 3 ✭
    edited October 2018 #8
    Thanks for the suggestions, everyone! I think I'm going to look to ultimately relocate the PV/battery. I can put it in a much more centralized location, run multiple loops, and cut the lengths significantly. The original length of the run was to place the battery/switches near the entry door (and avoid walking through the area in pitch black or fumbling around with a flashlight/cellphone). I think I might solve that problem with a door activated switch, or possibly a motion-activated light - just enough to safely illuminate the path to the other switches. I think this way, even 18AWG will work, and with multiple loops, I'll have more options on how to manage my usage.
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