hannesvn

Gave it some more thought. If I use a relay with a NO nd NC output, I could connect those outputs to my buzzer via a 2-way switch. That way I can silence the buzzer, and at the same time arm it for the next change in status.

This will work, and you can power the buzzer from the inverter output. Also install a switch to switch the buzzer off, or it will sound the whole time Eskom is off.
Others use an indicator light instead of a buzzer, and place it in the kitchen and family rooms which are most frequented.
When Eskom is ON, the light is OFF, and when Eskom fails, the light is ON.

Grid tie, aka grid export is a expensive exercise, so no, the wheel spinning backwards in this case gives you no reward. 
Your panels are picking up around 60-70% of your load from your graph, you only use the balance from the grid, this is your current benefit. 
Another 4 panels should keep you from using grid power. However a proper analysis of your power needs hasn't been established yet. 
Perhaps discuss your objectives and steps taken so far, as a means to guage where you are with your goals
At least your system has good building blocks, with extra panels and maybe 1 or 2 extra batteries being the question