[RE-wrenches] Back feeding battery inverter with grid-tie inverter

[Jeff Yago]

I know this subject has come up before, but there have been several different suggested ways to handle the problem and I was hoping for easiest.  We install lots of both on grid and off grid systems.  However, we have a client that will require a ground mounted array much futher away from the inverter and batteries in the garage than we normally would want to do since the voltage drop for this long distance would be excessive at low voltage even with large cables.  We have decided to locate a weather-proof sunnyboy grid-tie inverter at the array, and supply 120/240 volts to the critical loads sub-panel which is supplied by a Xanrex XW inverter and battery bank in the garage. I realize there are several design issues here when the grid is down,  since we want the grid-tie inverter to remain on but be blocked from the grid by the Xantrex inverter.  

The question is, what is the best way to keep a load on the sunnyboy since it will not be grid connected during an outage.  Also, is there a best way to feed the output from the Sunnyboy to the Xantrex that does not just power the sub-panel loads during an outage, but also will re-charge the battery bank through the Xanrex?

If you back-feed the Xanrex from the sub-panel it is normally supplying with output from the Sunnyboy, will the Xanrex see this input and think the grid is up and be able to re-charge the battery bank?

Will we need any separate transfer switches or power relays?

Thanks,

Jeff Yago

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