Small system dual inverter question?

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Saburai
Saburai Solar Expert Posts: 32 ✭✭
in Solar Beginners Corner #1
Hi Gang!

I'm considering running two inverters on my small off grid 12v system.
Is it reasonable to run two inverters on the same circuit? The main is a 2000w inverter/charger and secondary, a smaller morningstar 300w suresine. They would both be remote switched. This system is on a smaller RV that I have re-purposed the 30A A/C feed and junction box. To clarify, what I'm asking, both inverters will draw from the same battery bank and feed the same 30A circuit using the remote mounted switches to toggle between them as needed. Will there be problems back feeding if just using the remote switches? would I be better off and safer using manual switching? If so what do you recommend?

Many thanks,

Rich
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Small system dual inverter question?

    Whoa! Stop!

    You can not connect the AC outputs of two inverters together unless they are specifically designed to be 'stacked'.

    The DC side is not a problem, but the AC output would need to be isolated between the two; one inverter feeding one set of circuits the other dedicated to its own loads.

    The only safe way to switch between two non-synchronized AC power sources is with a transfer switch, and that is not included in your inverter remote control.
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  • Saburai
    Saburai Solar Expert Posts: 32 ✭✭
    #3
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    Re: Small system dual inverter question?

    Thanks for the verification!
    That's what I was thinking.
    Looks like a box and a pair of breakers is in order - or do yo have a better idea?
    Cariboocoot wrote: »
    Whoa! Stop!

    You can not connect the AC outputs of two inverters together unless they are specifically designed to be 'stacked'.

    The DC side is not a problem, but the AC output would need to be isolated between the two; one inverter feeding one set of circuits the other dedicated to its own loads.

    The only safe way to switch between two non-synchronized AC power sources is with a transfer switch, and that is not included in your inverter remote control.
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  • Saburai
    Saburai Solar Expert Posts: 32 ✭✭
    #4
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    Re: Small system dual inverter question?

    Some poking around came up with this 30A auto transfer switch:
    http://www.donrowe.com/PowerMax-PMTS-30-Automatic-Transfer-Switch-p/pmts-30.htm
    I called Don Rowe, which is where I sourced my inverter and after speaking at length with Patrick it seems that this might be a viable way to go. The 2000W inverter would be primary, kicking the suresine off the system when I turned on the remote for the 2000W unit.
    Thoughts?

    Many thanks

    Rich
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #5
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    Re: Small system dual inverter question?

    Yes, if you have a "two pole" transfer switch minimum (3 pole if you have have AC Neutral to Earth Bonding issues, i.e., utility power, generator power, two different AC inverters, etc.).

    I guess the other question, would you be able to run the small load from the 300 Watt inverter and run the "on demand" 2,000 watt load from the other inverter?

    The little MorningStar has both a 12 volt on/off remote input and a "search function" (looking for loads over >~6 watts)--Both are pretty sweet on not found often on 12 volt DC inverters.

    Also, the MorningStar takes around 6 watts when "turned on"... The transfer switch relay may take upwards of 6 Watts too... I.e., the relay may use about as much power as running the MorningStar--So the relay is not really saving anything.

    If you are looking for "backup" AC power if the 2kWatt inverter goes bad--That is another discussion.

    Also note that drawing 2 kWatts from a 12 volt battery bank--About the maximum I would suggest trying (lots of heavy copper wire for the DC bus).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #6
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    Re: Small system dual inverter question?

    I used to run a Suresine 300 and also a 1800 watt inverter off the same 12 volt bank. BUT!!!! The two inverters did not feed a shared distribution system. The Suresine was hard wired to certain loads such as lights and other small loads. It was also on 24/7. The bigger inverter was kept in Power Save mode and hard wired to the larger loads it was to run, such as water pump, microwave, washing machine band saw etc. Worked great this way.
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  • Saburai
    Saburai Solar Expert Posts: 32 ✭✭
    #7
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    Re: Small system dual inverter question?

    Thanks Wayne and Bill,

    The Morningstar will not be as "back up" for the 2kw. I would like to have them both available, on demand. Another and perhaps the primary reason I am considering the transfer switch is that this installation is temporary. We are seasonal residents here in Florida and purchased this slightly too small 25ft RV at a price we could not turn down. We bought it with the idea of selling it at a profit in a few months at the end of our season and buying a larger RV when we return next season. All of the off grid power upgrades need to be removable and have minimum impact on the rig as well as keeping my labor to a minimum. I'm able to eisily divert the main 30A service through the inverter/charger. Rewiring for the individual circuits is much more invasive and labor intensive...

    Many Thanks

    Rich
    BB. wrote: »
    Yes, if you have a "two pole" transfer switch minimum (3 pole if you have have AC Neutral to Earth Bonding issues, i.e., utility power, generator power, two different AC inverters, etc.).

    I guess the other question, would you be able to run the small load from the 300 Watt inverter and run the "on demand" 2,000 watt load from the other inverter?

    The little MorningStar has both a 12 volt on/off remote input and a "search function" (looking for loads over >~6 watts)--Both are pretty sweet on not found often on 12 volt DC inverters.

    Also, the MorningStar takes around 6 watts when "turned on"... The transfer switch relay may take upwards of 6 Watts too... I.e., the relay may use about as much power as running the MorningStar--So the relay is not really saving anything.

    If you are looking for "backup" AC power if the 2kWatt inverter goes bad--That is another discussion.

    Also note that drawing 2 kWatts from a 12 volt battery bank--About the maximum I would suggest trying (lots of heavy copper wire for the DC bus).

    -Bill
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #8
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    Re: Small system dual inverter question?

    Just to be sure you really understand - - you can not wire the outputs of these inverters together. Their AC outputs must NEVER be connected together, at least not until the 4th of July when everyone else is also setting off fireworks. :D
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #9
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    Re: Small system dual inverter question?

    Unless you use an AC Transfer Switch between the two inverters and your AC load.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Saburai
    Saburai Solar Expert Posts: 32 ✭✭
    #10
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    Re: Small system dual inverter question?
    waynefromnscanada wrote: »
    Just to be sure you really understand - - you can not wire the outputs of these inverters together. Their AC outputs must NEVER be connected together, at least not until the 4th of July when everyone else is also setting off fireworks. :D

    Understood, thanks!
    BB. wrote: »
    Unless you use an AC Transfer Switch between the two inverters and your AC load.

    -Bill

    Right, that's why I am considering the above linked AC transfer switch. Patrick at Don Rowe seemed to think that the draw was micro amps, but he was not 100%. I'll attempt to get in touch with Powermax and see if they have any idea what it draws.
    If both inverter remotes were turned off the transfer switch would be dormant, only causing parasitic when one of the inverters were powered up. If it's less than 4-5 watts, I might be able to live with it...

    Many thanks!

    So much good info here!

    Rich
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