Inverter direct off the solar panels?

cruiser guy
cruiser guy Solar Expert Posts: 87 ✭✭✭✭
in Solar Beginners Corner #1
We're out in Mokanji, Sierra Leone and we are constructing a childrens home which will have 12vDC LED lighting for the night time and a 12v - 230v 600w square wave inverter to allow the occupants to charge their cell phones. I'd like to be able disable the inverter at night to avoid drawing the batteries down too far. To force shut down of the inverter I was thinking of hooking the inverter to the solar panels direct in parallel with the charge controller. That way if there is no sunlight the inverter cannot be used.

Does anyone see an issue with this arragement?

I plan to have a nominal 12v coming from the panels.

The reason to shut off the inverter is that here charging phones is a business and "free" charging means there will ALWAYS be something plugged in to charge.
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  • Seven
    Seven Solar Expert Posts: 292 ✭✭
    #2
    Re: Inverter direct off the solar panels?

    Unless you are grid tied, you cannot hook the inverter directly to the panels.

    I don't know of any square wave inverter that is still in production. There are modified square and then there are sine wave.

    The simple solution would be to turn your inverter off when you didn't want it to be used. You could probably put a timer on the output line to have the same effect, similar to how some places control their parking lot lights. If there are certain outlets that are used for the phone charging you could manage it that way too.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #3
    Re: Inverter direct off the solar panels?

    No, you will not be very successful trying to power the off grid inverter directly from the solar panels. First, solar panels have no surge capacity, and second the inverter will probably over voltage (solar panels go to 17-22 volts when they are not loaded--which will either cause the inverter to shut down, or cause damage).

    You will need a heavy power switch (even small inverters have lots of current):
    • 600 watts * 1/0.85 inverter eff * 1/10.5 volts cutoff = 67 amps
    And you should design the wiring (and fuse/breaker, if any) at 1.25x the continuous current:
    • 67 amps * 1.25 safety factor = 84 amp minimum fuse/wiring rating
    Lastly, I assume that MSW inverters are probably very common in your application (MSW inverters are much less expensive), but I would suggest a TSW inverter for charging small electronics. MSW can be very hard on small transformers and power converters:

    All About Inverters
    Choosing an inverter for water pumping

    There is a really nice 300 watt (600 watt surge for 10 minutes) 12 VDC inverter from MorningStar--Includes "search mode" (only turns on full if > 6 watt load... And includes a "remote inhibit" function (a 12 VDC on/off signal input).

    There are both 120 VAC 60 Hz and 230 VAC 50 Hz versions...

    Otherwise, you should get a heavy duty DC rated switch (don't know what is available in your region)... But one hand source here are from Marine Suppliers:

    Blue Sea Battery Switch 1-2-OFF 350 Amp
    wind-sun_2185_970775 Blue Sea 9001e 350 Amp Battery Switch Make-before-break contact design allows switching between battery banks without power interruption Ignition protected - Safe for ... mounting Meets American Boat and Yacht Council (ABYC) requirements for battery switches 3/8"-16 tin-plated copper studs for maximum ...

    Even though they are DC rated--I would recommend turning of your AC loads first to lessen DC arcing (remember, 12 VDC has 10-20x the current vs your AC loads--and DC is much better at arcing than AC).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    #4
    Re: Inverter direct off the solar panels?

    It most likely is not a squarewave inverter. As to the best of my knowledge they have not been made for more than 20 years,, so ineficient and destructive of most small power supplies.. will kill a cell phone battery charger in minutes
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #5
    Re: Inverter direct off the solar panels?

    Just to be clear, a square wave inverter is either at +V or -V and that is it.
    [FONT=Fixedsys]
    ___     ___
   |   |   |
___|   |___|
[/FONT]

    A Modified Square Wave has a "shoulder" at 0 Volts:
    [FONT=Fixedsys]
      ___         ___
    _|   |_     _|
___|       |___|
[/FONT]

    As I posted earlier, you can read here about why you should care:

    All About Inverters
    Choosing an inverter for water pumping

    And it is the old 80/20 rule... 80% of devices will probably work fine, and 20% may over heat and/or have early life failure. The difficulty is that one cannot always tell which device falls into which category.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • cruiser guy
    cruiser guy Solar Expert Posts: 87 ✭✭✭✭
    #6
    Re: Inverter direct off the solar panels?

    The reason why I'd like to run it off the panels is to render it useless outside of the time the panels might charge the batteries. If it is not rendered useless outside of that time someone WILL use it till the batteries are fully depleted, that's just a fact of life here.

    Maybe I need a simple light operated switch? I'll look into the Morningstar inverter as well.

    It must be a modified square wave inverter then as I bought it in Germany about 6 months ago. It is NOT a true sine wave, that is for sure.
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  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    #7
    Re: Inverter direct off the solar panels?

    many charge controllers have an AUX switch built in to do exactly what you want the switch is used to turn something on after sunset and ooff at dayliight, usually a night light. but it can be used with a relay to turn everything off from the batteries

    keep the batteries in a locked enclosure
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #8
    Re: Inverter direct off the solar panels?

    even if a controller does not have an aux to it you can still power an external relay's coil by solar observing the relay's coil max voltage specs and allowing for enough current capability to pass through it's contacts to properly power whatever you have in mind. no solar power to the coil means no energized contacts so it should be in a normally off configuration and this adds no tare losses while there's no sun and only the power required to operate the relay's coil as the tare loss while there is sun.
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  • halfcrazy
    halfcrazy Solar Expert Posts: 720 ✭✭✭
    #9
    Re: Inverter direct off the solar panels?

    Depending on what you use for a Charge Controller the answer may be there. For instance our Classic has a Daylight mode in the Auxiliary contact this would power a relay to keep the inverter alive or as Niel said power a relay allowing ac to pass to an outlet.
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  • cruiser guy
    cruiser guy Solar Expert Posts: 87 ✭✭✭✭
    #10
    Re: Inverter direct off the solar panels?

    Thanks guys, the relay idea should work. A simple automotive relay is all I'll need!
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