A little back story...
This system has evolved to where it is now, with cobbled together parts on a budget. I picked up a dozen M215 for like 50.00 each on ebay, with the thought that they would be better that the cheap chinese grid tie inverters. They are a total steal at 50.00 each, and work great with one panel. They even work great in a battery discharge application, drawing about 215 watts for as long as the battery holds out with not much heat generated. Only problem is you have to have access to 240V split phase to hook them up to.
The system is mostly battery based, using a pair of 5.3 kw Tesla modules in parallel to charge my Nissan Leaf through an inverter. Thing is in the summer, 9 X 250 W panels produces more power than I can use for charging the leaf, so therefore the grid tie to use the excess power. The house has a grid tied system, so turning the meter back a bit more is not an issue with the utility.
Bill, thanks for your carefully considered response:
1. Yes got that.
2. Actually it works just fine. I have had 1000 watts of panels in parallel hooked up to one inverter. It just clips the power at 215 watts. It even works connected directly to the 22V the tesla module. Barely even gets warm.
3. OK, I didn't think of the MPPT of each inverter trying to adjust the operating voltage and fighting each other. I can see that being less than optimal, but why does it just kill both inverters instantly, and permanently? I'm trying to figure out why, and if anything can be done to isolate them from each other, short of a pair of isolated DC-DC converters.
4. I am just trying to feed excess power back into the grid once the batteries are charged.
a) The way the system is set up for battery charging is 9 panels, 3S, 3P, so this would require a total re do, and would not work when the grid is down, and I use it for backup power.
b) This could work, but I would have to switch between parallel/series wiring the panels to get the voltages right, as well as switching between the charge controller and grid tie, so really complex switching.
Right now I am switching one M215 using the charge controller's "waste not" setting, it switches the AC on to the M215 when the batteries are floating. The DC side is permanently connected to the batteries, this works fine, but will not be enough in the summer.
c) No room left on the shed roof for more panels.
The simple thing would be to purchase a hybrid inverter, but that would require big $$$
The other way would be to purchase a single larger grid tie inverter, but the good ones need a minimum of 150V so would require series parallel switching again, as the charge controller maxes out at 150V.