Just to be clear, are you saying that when the bill comes your Kw are taken off the total used. Not the 'cost' of them?
For example if you used a total of just say 100kw for the bill and you fed back 95kw you are only billed for 5kw?
I thought it would be you used 100kw and the bill should be say $200 they then work out that 95kw times 30c = 28.50 so your bill would be $171.50
If it is the first then what you are saying sounds much better to me. And building a system to your usage makes sense. Provided you are only billed for what you use over what you feed back.
So essentially the tarriff is for everything OVER what you use. I hope i am getting this right. 
Actually you use what you make as you make it anything over goes back to the grid and you will be credited for it, so if your system is making 20kw per hr and your current usage at that time is 15kw then 5 kw will be fed back and you will get that off your bill. If your usage is 25kw then you will only be charged for the 5kw over what you have made.
Thanks for you info rustalan, that is exactly how I had heard it worked, so to get the biggest benefit you have to be home using power (or have a big swimming pool pump sucking power) during the daytime whilst the sun is out, if like most households your power is used in the morning and evening, it is not so good you are effectively, after the 3 year honeymoon period finishes, getting 6-8 cents (or whatever they decide the feed in tarriff is worth) off your kwh rate
