What is the difference between a 12V, 24V, 48V solar System?

hmong2017hmong2017 Registered Users Posts: 35 ✭✭
For an off grid Solar panels, breakers, controller, batteries and inverter.... Whats the REAL difference to choose from a 12V, 24V and 48V system? Why do others choose a specific system vs others? 

I only see that they all produce electricity and makes 110 and 120volts.... 
Is bigger ones like 24v and 48V just simply charge batteries faster or what? 
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  • EstragonEstragon Registered Users Posts: 3,843 ✭✭✭✭✭
    edited May 12 #2
    The main difference is the size of loads you need to run.  Running a couple of 15a(AC) 120v loads means > 300a(DC) on a 12v system, requiring heavy wire etc., which gets pretty impractical. Running on a 48v system the same loads would be 1/4 the amps, so ~75adc.

    A 12v system would also require 4x the charge controller capacity.  Eg. 1200w would be 100a out from12v controller, likely needing 2 units.  25a out from a 48v controller easily handled by one.  A 60a controller handles ~ 60x12v=720w of panel, same 60a controller is ~ 60x48v=2880w on a 48v system.

    So, for larger systems, higher nominal voltage makes more sense.  Offsetting this is, for smaller loads and systems, lower system voltage tends to be more efficient and cost effective.

    There is no best choice, it depends on loads.
    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
  • BB.BB. Super Moderators, Administrators Posts: 29,333 admin
    As Estragon says, the higher the loads, generally, the higher the voltage for the battery bank.

    For example, we say that roughly 100-150 Amps for the battery bank current, then for a 12 volt bank, I would suggest the largest AC inverter or DC loads would be roughly (ignoring losses and variable battery bank voltage) in the range of:
    • 100 amps * 12 volts = 1,200 Watts maximum suggested
    • 150 amps * 12 volts = 1,800 Watts not to exceed if avoidable
    For 24 and 48 volt banks:
    • 100 amps * 24 volts = 2,400 Watts max suggested
    • 150 amps * 24 volts = 3,600 Watts not to exceed...
    • 100 amps * 48 volts = 4,800 Watts
    • 150 amps * 48 volts = 7,200 Watts not to exceed
    In reality, if you heavier (and paralleled) copper DC cabling, you will find 12, 24, 48 volt inverters that are larger than the above recommendations, and they can work... But exceed the max suggested is better done with higher voltage battery buses (24 and 48 volts).

    Sending 100's of Amps at 12 volts--Really only leaves you about 0.5-1.0 volts of drop on the DC  battery bus for a "workable" system... For 24 volts, 1-2 volts, and 48 volt 2-4 volts drop--So you have more "headroom" with the higher bus voltages (and allows a larger DC load if needed and properly designed).

    Similar issues for charging your battery bank... For example a good quality MPPT controller can output 80 amps... At 12/24/48 volts. So, a higher voltage battery bank means a larger solar array could be supported... For example, our rules of thumb would suggest the maximum array for an 80 amp charge controller:
    • 80 amps * 14.5 volts charging / 0.77 panel+controller deratings = 1,506 Watt maximum "cost effective" array
    • 80 amps * 29.0 volts charging / 0.77 panel+controller deratings = 3,013 Watt maximum "cost effective" array
    • 80 amps * 58.0 volts charging / 0.77 panel+controller deratings = 6,026 Watt maximum "cost effective" array
    The same $600 controller can support much larger arrays with higher battery bus voltages.

    Anyway, details matter here. The above are good rules of thumbs to estimate a workable system without going into a bunch of detailed design issues. But they can be modified as needed.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PhotowhitPhotowhit Solar Expert Posts: 4,957 ✭✭✭✭
    Normally the first thing that we point out is the wire sizing to run high DC current to the battery bank. Had someone the other day have a 15,000watt 24 volt inverter... So a quick look at the NEC ampacity charge and some quick math at a full 24 volt and the wire size required is 1500mcmil often 4/0 is compared to the size of a finger, 1500 mcmil would be more comparable to your wrist!

    If you are looking for inverter sizing, you will find that inverters that are UL1741 (for home use) rarely are far from the sizes Bill has mentioned above. Magnum makes a 2800 now 3000 watt inverter for mobile use and it only carries UL458 (mobile use) They do make the MS2000 12 volt which I think is rated for 2000 watts continuous.  They also make a 4000 watt 24 volt, largest I'm aware of in 24 volt. I believe there are a couple 8000 watt 48 volt inverters, Outback's Radian, and Schneider? larger than that and you would have to 'stack' inverters. So just to be 'legal' you may find these ranges.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
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