Reading the doc, it looks like when the GT output (+3KW) exceeds the XW charging capacity of ____ , magic smoke gets released from somewhere.

And I had stocked up on 500W, 400V DC-DC 28V converters to charge a makeshift battery bank.

Well I guess its good that I only have 3KW worth of positive ground Sunpower Panels. - And Solar Guppy, thanks for th info. Very enlightening.

I don't know about the XW or other inverters specifically, but assuming you have AC input and and inverter that automatically transfers the load when AC input fails (and restores the AC in to AC out via transfer switch) and turns off when the AC input is back on again--I see no reason that you could not push through grid tied inverter power back to the AC input (utility line).

There could be an issue with limitations of the AC battery charging circuit. For example, many inverter chargers allow you to set a maximum current through the inverter... Say 10 amps. If you have zero load on the inverter, then 10 amps (120 vac) is available to charge the batteries. If your inverter draws 5 amps, then only 5 amps would be available to charge the batteries (etc.).

What happens when your Grid Tied Inverters starts pumping power back through the line? It may look like a "load" (assuming the inverter/charger does not detect direction of current flow) and start to limit the amount of power available to charge the batteries.

This is probably not the worst thing in the world--at worst, your battery charging may be reduced during 9am to 3pm--and shift your maximum charging to off-peak times when the grid is up. In my case, that is actually a good thing as I have Time of Use metering and this would shift my optional power usage to off peak times...

In terms of safety, using the GT inverter shutdown to control battery charging voltage is OK--but I would probably recommend that a second dump controller be installed on the batteries too to prevent overcharging in the event the primary battery monitor or GT shutdown circuit failed or the control cable were cut (you might want to hook up an alarm (light/bell) on the Dump Controller to indicate a primary circuit failure).

Basically, 99% of the time, a Grid Tied / Off Grid Inverter/Charger setup like this is going to trickle charge (and equalize) based off of the function of the internal battery charger in the off-grid inverter (or separate AC battery charger if you go this route).

For the 1% period of time when you are off-grid (utility lines failed), then you will be using the charge through the inverter back-feed process. Having a second dump controller might be a bit of overkill--you could probably achieve similar results by just using a over voltage alarm and manual kill the GT inverter if you are home most of the time. Would depend on size of solar array and battery bank (might just be an unexpected battery equalization function).

Where I see more work needing to be done is with generator backup... You could not want to feed much (or any) power back to the generator and unless is it an Inverter/Generator, the AC power would probably not be stable enough to allow the Grid Tie inverter to function (frequency out of range failure?). Hence, my earlier suggestion of using the generator to directly recharge the batteries and forget the GT/Inverter-charger/Generator interaction.

I still wonder about NRTL/UL/Building code violations since none of this has probably been tested/approved for this type of usage. You want to make sure that everything (fuses, equipment rating, wire runs, batteries, etc.) are all properly designed and interconnected. Don't want somebody killed or loss of insurance over this "off-spec." usage of equipment.

-Bill

Thanks for the pointers, But as I have discovered. Codes and inspections dont seem to be much concern in this area, and the insurance company knows what we are doing, they gave us the loan! We even asked the town and utility co. and they told us we dont need any inspections or permits. So I will do my best to be sure that the system is installed in a safe manner. Looks like Xantrex is going to be my choice for inverters and hardware after all though, which Im happy about. And the GT inverter costs close to the same as the 3 MX60s or 3 XW controllers I would have needed anyway if I went battery connected. I figure the overal costs of the system should be the same. I will just need to keep an eye on it. And we wont be using a generator. The whole Idea is to have adequate backup for at least 1 day before batteries need recharging, and to do it all RE.

Just an Update to this thread, the system is now online and has been generating power for the past week. We get between 15 and 21 kwh per sunny day, but its been cold so that may change. Thanks for all the help guys!