Using a 12VDC/120VAC inverter on a 24VDC Solar system

LostOnFiji
LostOnFiji Registered Users Posts: 3
I have a 24VDC solar system that charges 2 gel cells in series.  This runs a 24VDC/240VAC inverter.  I also need to have 120VAC power.  I have a 12VDC/120VAC inverter and I have a 24VDC/12VDC converter rated at 40 AMPs.  I also have a few 12VDC powerpoint and a 12VDC water pump.  I'm sure the DC/DC converter will work with the powerpoints and the water pump, but I am concerned about hooking the 12VDC/120VAC inverter to that same circuit.  It seems as if it should work.  Are there any "gotchas" I need to worry about?

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    edited December 2019 #2
    What is the Wattage of the 12 volt inverter? Is it a PSW or MSW inverter?

    40 amps * 12 volts = 480 Watts... And most inverters can support ~2x (or more) surge current. And if your water pump is 10 amps, that leaves 30 amps / 360 Watts left for the inverter...

    You are also looking at double conversion (24 VDC to 12 VDC to 240 VAC). So, that is an extra 10-15% losses.

    Overall, I would suggest buying a 24 VDC input inverter instead just to keep things simple--Unless the 240 VAC inverter is a a very small unit.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    edited December 2019 #3
    Maybe a silly question, but is the 240v inverter just 240v (line to neutral), not split phase (L1-N-L2)?

    Also, maybe a non-issue, but 40a is kind of material in the context of what I'm assuming is a ~75-100ah@24v bank.  The 120v inverter may cause a "ripple" on its DC supply side, and I'm not sure if/how that might affect the 24/12v converter operation. 

    A 240v/120v step down transformer (or 24v/120vac inverter) would seem to me to be a better solution.
    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
  • LostOnFiji
    LostOnFiji Registered Users Posts: 3
    Thanks for the responses.  The 120VAC inverter is 1500W so it would overwhelm the DC/DC converter.  I agree that the "best" solution is to get a 24VDC/120VAC inverter.  Price alone dictates this choice.

    FWIW: I tried to connect the DC/DC converter to the output of my solar controller.  That was a mistake.  Connecting it to the batteries though is working just fine.  The amp requirements for the 12VDC systems (without the 120V inverter) don't come close to the 40A the converter make available.

    So, just to complete this thread:
    No you cannot use a 40A 24VDC/12VDC converter to power a inverter that will output 1500W.


  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    Just to clarify a bit... Unless you are making a "batteryless" solar power system (it can be done), in reality the Battery is the "heart" of your system. If you do all of your design assumptions based on your battery bank's capabilities (assume it sources all current, and sinks all charging current)--You are off to a good start.

    The battery bank pretty much sets the system bus voltage (i.e., 10.5 volts dead to ~15.0 volts hard charging).

    Solar Charge Controllers are not really "voltage regulators"... The provide current to the battery bank, based on estimated state of charge and DC load current--And the battery "averages" it all out to the bus voltage.

    If you take a solar charge controller and attempt to run it without the Battery to "buffer" the output voltage, the solar charger cannot give you a 14.4 volt "stable output".

    Similar with a car alternator--It will output 50-100+ volts without loads/battery to buffer its output voltage.

    And for folks that ask, you cannot split 12 volts off a 24 volt battery bank (12 volt inverter, 24 volt charging)... It will "unbalance" the series connected batteries (the low 12 volt will be under charged, and the high 12 volt will be over charged)...

    There are ways to make banks like this work (common for a single battery bank used on a 24 volt bus chassis and 12 volt inverter/DC house power)... But that is for a different discussion (and is not usually a cheap solution either--More hardware).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset