SMA reckon for their 6kw inverter 11kW AC coupled.
For the 6kW OzInverter we reckon about 8kW. Its all about handling the back charging to the batteries.
How big is big?
We here are going to about 15kW for AC Coupling with PV.
We have 8kW DC coupled to the 48v 1300ah batteries and keep them in tip top condition, using Tristar diversion controllers for the Turbines and Tristar MPPT and a 200 Classic for the PV Trackers.
The Classic aux contacts are used as a fail safe trip out on the AC coupling GTI's at the 5 installations. So the classic goes to float charge and switches off the GTI's sequentially.
I purchased a very well known manufacturers Inverter, who insisted on lots and lots of expensive auxiliary control equipment to make it work, not mentioned in the Inverter specs/brochure. I sent it back, got my money back and made a simple, robust, and very cost effective OzInverter.
Battery charging. ....... The OzInverter will take GTI's AC coupled to the Inverters AC output and back feed it to the batteries. When the batteries start to fill they push back and the Inverters AC voltage rises slightly, say 12 volts above 230vac, and this is normally enough to trip out the GTI's. So my fail safe operated by the Classic never has the need to work.
I have a BigOzInverter 10kW to 15kW and can handle fantastic loads for short brief periods. And this will replace the present 6kW to 15kW OzInverter.
GTI's for me are relatively cheap, New Old stock or second hand discarded when the installation they were serving was upgraded. Mine are all under 3kW I use SMA SB's toroids, as I have internal codes for modding them.
Control equipment ....... once your locked in with all that expensive gear, the manufacturer seems to have you by the short and curlies.
All my control gear is under $3000 for a 23kW system.
Every time I have a nice glass of wine I raise a toast to 'Oztules', the man who lifted me out of manufacturers reliance.
Ps, As I understand it 'Oztules' is about to explore a simple, robust, and cost effective, Solar charge controller.