12v, 120v, 240v

morpho
morpho Solar Expert Posts: 176 ✭✭✭
So I have been reading and looking in the archives and have not found a clear reason why in an off grid system there should be a preference for 12, 120, or 240V.

Anyone have a reasonable explanation in the case of an Off grid system where the transmission distances are not far, the loads are LED lights and computers for the most part. (for the 12V I would have a small pure sine inverter for the computer.) No refrigerator, no welding equipment, etc.?

Thanks.
11.5 kw array  -  3 - midnite solar classic 200's   -  Magnum MS-PAE Inverter  -  Magnum ME-RC50 remote  -  ME-BMK batt-monitor  -  8 DEKA Solar GC15 230amp hour (48v) - Yamaha ef3000ise - 1 confused look on face

Comments

  • Windsun
    Windsun Solar Expert Posts: 1,164 ✭✭
    Re: 12v, 120v, 240v

    There are a lot of off grid systems that are much larger than what you envision.

    The largest we have sold was over 24kw of panels.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: 12v, 120v, 240v

    My two cents--I like to limit the DC battery current to around 100 amps maximum...
    • 12 volts * 100 amps = 1,200 watts maximum load
    • 24 volts * 100 amps = 2,400 watts maximum load
    • 48 volts * 100 amps = 4,800 watts (normal RE battery voltage limit)
    For sending power any long distances--look at the supported voltage drop:
    • 12 volts - 10.5 volts = 1.5 volts drop maximum
    • 24 volts - 21 volts = 3 volts drop maximum
    • 48 volts - 42 volts = 6 volts drop maximum
    • 120 VAC - 3% = 3.5 volt drop nominal
    • 120 VAC - 110 VAC minimum = 10 volt drop nominal
    • 240 VAC - 3% = 7 volt drop nominal
    • 240 VAC - 220 VAC minimum = 20 volt drop nominal
    And if you look at the power that you can send any distance--Say 15 amps on 14 AWG wire for 100' (200' round trip).

    For a 12 volt circuilt:
    • 14 AWG, 100' distance, 15 amp = 9.1 volt drop
    • 14 AWG, 100' distance, 2.5 amp = 1.5 volt drop
    • P= V*I = 10.5 volts * 2.5 amps = 26 watts
    Same thing, 24 volts:
    • 14 AWG, 100' distance, 5 amp = 3 volt drop
    • P= V*I = 21 volts * 5 amps = 105 watts
    Same thing, 48 volts (limited to 10 amps max because of voltage drop) [fixed next two equations]:
    • 14 AWG, 100' distance, 10 amp = 6 volt drop
    • P= V*I = 42 volts * 10 amps = 420 watts
    Now 120 VAC (limited to 15 amps max for 14 awg) [fix mistakes here]:
    • 14 AWG, 100' distance, 15 amp = 9.1 volt drop
    • P= V*I = 110 volts * 15 amps = 1,650 watts
    240 VAC (limited to 15 amps max for 14 awg):
    • 14 AWG, 100' distance, 15 amp = 9.1 volt drop
    • P= V*I = 220.1 volts * 15 amps = 3,302 watts
    The above may or may not be to code--but shows how choice of transmission voltage can dramatically affect how much useful power you can get out the end of the wire.

    -Bill

    PS: I had the 48 and 120 Volt entries messed up... Sorry. :blush:
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RandomJoe
    RandomJoe Solar Expert Posts: 472 ✭✭✭
    Re: 12v, 120v, 240v

    Another possible consideration is the cost of end devices. Sure, you can get light bulbs and appliances that run natively off 12VDC, but they are almost always much more expensive than 120VAC equivalents (thanks to economies of scale in production) and often much more limited in variety (appearance, size, style, whatever).

    Inverters have become quite efficient as well, so you aren't losing much power to inverter losses by going that route either.
  • morpho
    morpho Solar Expert Posts: 176 ✭✭✭
    Re: 12v, 120v, 240v

    hmmm?
    BB....
    I have read your post a few times .....and I think I will need to look at it some more actually.

    Randomjoe:
    I see the point in that for sure.

    Windsun:
    I'm sure there are.....

    As far as I can figure out the distance from the panels to the batteries will not be much more than 15' so my guess is that this gives me more options....?
    11.5 kw array  -  3 - midnite solar classic 200's   -  Magnum MS-PAE Inverter  -  Magnum ME-RC50 remote  -  ME-BMK batt-monitor  -  8 DEKA Solar GC15 230amp hour (48v) - Yamaha ef3000ise - 1 confused look on face
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: 12v, 120v, 240v

    Morpho,

    I was trying to use real numbers (14 awg wire, working voltage drop, real wire resistance) and show how different bank/AC voltages dramatically affect sending energy 100 feet. Shows the maximum amount of energy each voltage would be able to deliver to a load at the end of a 100' foot run.

    Hmm... I guess that mistake I had in the 48 and 120 Volt transmission calculation would confuse the issue (I reversed the two voltages in the equation)--let me fix. :blush:

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Kamala
    Kamala Solar Expert Posts: 452 ✭✭
    Re: 12v, 120v, 240v

    Brings to mind the Edison vs. Westinghouse war.

    DC good for VERY short runs. Especially with the magnitude of work modern devices are expected/required to do. No power factors or phase angles to consider. IOW, no imaginary numbers, such as i.

    AC great for efficient, economical, profit generating distribution.

    Is Westinghouse responsible for the plethora of wallwarts stashed away in long forgotten nooks and crannies of my basement shelves?

    Adding this from article above:

    "In January 1998, Consolidated Edison started to eliminate DC service. At that time there were 4,600 DC customers. By 2006, there were only 60 customers using DC service, and on November 14, 2007, the last direct-current distribution by Con Edison was shut down. Customers still using DC were provided with on-site AC to DC rectifiers.[19]"

    Who knew?
  • morpho
    morpho Solar Expert Posts: 176 ✭✭✭
    Re: 12v, 120v, 240v

    BB....you could have put in a series of nonsense numbers and I would have still not notice there was something wrong in what you posted.
    Numbers have never been my forte. If there is such a thing as Numerical Dyslexia...I have it.
    Thank god there are people who do have this ability otherwise I'm sure we would all still be banging rocks together in a cave somewhere....aaaaah the good old days.

    Kamala,
    Are you saying - in your opinion - an off grid system where the transmission distance is minor using DC may be a more logical choice?

    I think what I am trying to get at is:
    without getting into too much technical detail if you are off grid and the runs of cable distance is not far and the loads are moderate....some lights (LED), a computer (monitor turned down) maybe a small efficient refrigerator (with plenty of breathing room around it etc.) what is the logical thing to do in regards to system voltages?
    Panels, battery configuration, inverter, wiring in the house.

    I'm sure there will be a long string of numbers about to follow.......
    I think I'll take an Advil now in preparation.

    As you can tell I'm still lost.....any body have a pretty picture for me too look at? ;)
    11.5 kw array  -  3 - midnite solar classic 200's   -  Magnum MS-PAE Inverter  -  Magnum ME-RC50 remote  -  ME-BMK batt-monitor  -  8 DEKA Solar GC15 230amp hour (48v) - Yamaha ef3000ise - 1 confused look on face
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: 12v, 120v, 240v

    I guess I was trying to show, that for every time you double the voltage, you can transport 4x as much power.
    • Power = V^2 / R
    Short question and answer:
    1. What are the "native voltages" of the loads (12 vdc, 120 VAC, both, etc.)
    2. What is the current/wattage required by your loads
    3. What is the distance between the battery bank and the loads
    4. Where will the solar panels be placed relative to the charge controller + battery bank.
    Basically, 12 volts is difficult to use. Requires heavy cables and short wire runs to do anything useful (requiring more than 100 watts of power).

    If you have the option, it is almost always better to go with the highest voltage you can justify. Wires are much smaller in diameter (save copper $$$) and voltage drops are less (and matter less). Also, battery voltage varies quite a bit (from 10.5 to 15.5 volts)--some DC electronics, lights, and such really don't like that wide of range (for example, reports of laptop car adapters failing on solar RE systems).

    If you can use 120 VAC for all of your loads (i.e., wall warts and such)... Then you only have one low voltage / high current interface to worry about (battery bank to inverter). The rest of the loads will be at 120 VAC, use standard home wiring techniques, and such.

    Yes, you will have the inverter losses (which can be 8 watts idling, to ~85% efficient)--But the common power (120 Vac) and the ability to send 120 VAC around the home and out in the yard with a stand extension cord is probably worth it.

    Similar issues apply to battery bank voltage... Only use 12 volts if your overall needs are small (less than 1,200 watts peak or so).

    For larger systems, use 24 or 48 VDC for the bank.

    For solar arrays greater than 400 watts and/or have a long run from array to the battery shed, use a MPPT type charge controller. Allows you to run much higher voltage from the array (upwards of 100 VDC), to use smaller gauge wire (lower current). The MPPT controller can down convert to your bank voltage (12, 24, 48 VDC).

    Notice that MPPT charge controllers are rated on Amps Output:
    • 60 amps * 14.5 volts charging = 870 watts of solar panels
    • 60 amps * 29 volts charging = 1,740 watts of solar panels
    • 60 amps * 58 volts charging = 3,480 watts of solar panels
    So--as you increase bank voltage--the one charge controller can manage 2x or 4x the number of solar panels... So, for larger systems, by increasing bank voltage, you can save on the number of charge controllers required to support larger PV arrays.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • morpho
    morpho Solar Expert Posts: 176 ✭✭✭
    Re: 12v, 120v, 240v

    This time you used more words and less numbers BB.
    Bless you.

    I get it.

    ahhhh....I can stop popping the advil.

    I'm sure I will come up with some other inane question shortly.

    Thanks once again.
    11.5 kw array  -  3 - midnite solar classic 200's   -  Magnum MS-PAE Inverter  -  Magnum ME-RC50 remote  -  ME-BMK batt-monitor  -  8 DEKA Solar GC15 230amp hour (48v) - Yamaha ef3000ise - 1 confused look on face
  • Kamala
    Kamala Solar Expert Posts: 452 ✭✭
    Re: 12v, 120v, 240v
    morpho wrote: »
    ...

    Kamala,
    Are you saying ...

    I was just bloviating.