but there's no general market atm for such inverters. so building lots & lots of lower voltage packs & using low voltage solar targeting modules, which are everywhere, works out.
but it's for sure not as good & makes less sense.
repurposing motor controllers is an interesting notion. but I'd probably rather diy than try to circuit bend that. big awesome mosfets &c are cheap. motor controllers have to deal with inductive loads (big coils) so they tend to have slow slopes. which matches up ok for low 60Hz. but it's still be an effort to repurpose & reuse. just redoing the inverter seems like the win. don't even try to find out what design parameters don't line up between motor drive & inverter drive, don't spend the time, for something that won't work as well as having the right thing.
You're probably right that just engineering a specific solution or ordering them from a supplier that makes such things is probably less hassle and possibly cheaper in the long run.
I'm currently doing an EV conversion. I'm amazed that you can buy a box that's about ten or fifteen pounds or so for a couple thousand dollars that can handle about 90 kilowatts of power (in short bursts), converting around 100-200 volts to AC power to drive a motor. That's probably enough to power a whole residential block by itself, and yet it's not even a very powerful controller compared to what you find in a high-end OEM EV. I'd expect anyone building a large scale grid backup battery storage system to have access to inverter parts at least that good and at least that cheap, and if they don't they could afford to design it themselves.
[1] https://ai.googleblog.com/2016/02/and-winner-of-1-million-li...
Actually, that sounds just about right, if you're working with modules rather than full packs. Tesla has various sizes of battery module, but 57 volts is one common size. (I think a typical model S has somewhere around 14 of them.) So, one microinverter per module would be fine. LG Chem also makes a module that's about 60 volts. (I think it's made up of around 16 pouch cells in series.)
Of course, if you're doing this on a large scale, you could just have any kind of inverter made that handles whatever voltage you want.
On the other hand, if you're using full packs you have the possibility of recycling motor controllers (also sometimes called inverters) from cars, which are designed to send 3-phase power to an AC motor, as a regular electrical system inverter running at fixed frequency. (Maybe fake the motor position sensor data so it always thinks the motor is spinning at a constant 3600 RPM, or 60HZ.)
I don't know if modern motor controllers output a real sine wave or if they just output a square wave. I'm guessing it's probably the former, otherwise I'd expect you'd get a lot of extra noise and vibration.
You could also re-use the car's original charger (the onboard part that converts 110V or 220V AC from a level1/level2 charger to DC at pack voltage). On the other hand, if the input power for charging is DC at the appropriate voltage to begin with, that part isn't needed.