I had heard stories that the original SMA 6000 inverters were unreliable but have not got any first hand experience of this.

The fire alarm problem sounds as if the Voltage is dropping below the threshold that the controller is happy with and the alarm is triggered by the on-board battery backup. I have a fire alarm that does exactly this whenever we get a grid outage. The alarm is just behaving as designed.

Is there any measurement and logging of Voltage and Frequency that you can put in place to get a better idea of what is going on?

ted