Dumb question that I can't find the answer to

azrc
azrc Solar Expert Posts: 43 ✭
in Solar Beginners Corner #1
Can I parallel a grid tie inverter with an off grid inverter? It kinda looks like that can be done with the SMA inverters, but I'm not sure.

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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Dumb question that I can't find the answer to

    Let's say it's not a good idea.
    Neither unit was designed to be connected that way.
    Which is not to say that some haven't done it. If the power consumption is high enough, apparently, it can work. But remember that a GT inverter is designed to put out all the Watts it possibly can, and "assumes" any surplus not used by the house will be taken on by the grid. Whereas an off-grid inverter is designed to supply Watts according to demand, up to its maximum limit.

    The SMA Sunny Boy is a special case: they actually make an additional unit that allows the GT inverter to "island" (look for SunnyBoy Island) and function as an off-grid inverter. Sort of. Basically it provides the necessary synchronization information the GT is looking for in the grid.

    But as GT inverters periodically "test" for grid presence and those "tests" can wreak havoc on an off-grid inverter we're back to the original "it's not a good idea." You could burn up $4000 in inverter here. :cry:
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #3
    Re: Dumb question that I can't find the answer to
    Cariboocoot wrote: »
    The SMA Sunny Boy is a special case: they actually make an additional unit that allows the GT inverter to "island" (look for SunnyBoy Island) and function as an off-grid inverter. Sort of.

    I think that "sort of" might give the original poster the wrong idea. The Sunny Island and Sunny Boy were designed to work together reliably and there are many large installations that use them in this configuration. SMA's _only_ DC coupling option is an overpriced 40A MPPT charge controller, yet their off grid kit like the 5kW sunny island supports clusters up to 300kW - the only way to build systems of that size (using purely SMA kit) is to use Sunny Boys.

    The caveats:
    - Battery charging is less efficient than a DC based charge controller
    - Powering AC loads directly from solar is slightly more efficient
    - Not all sunny boys are supported, check the sunny island manual for a list
    - Sunny boys are about twice the price of a comparable DC charge controller
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