It all depends on your bill, and more importantly, your usage at one time! Recently, a disturbing trend of installing larger systems than are needed has been growing. It's been brought on by reps trying to get a bigger sale and make more money, plain and simple, and is not good enough.
This week, we saw a quote for a 10kw system, and the bill was $350. If the customer had 3-phase power it could have been a good idea, but the rep didn't even check what sort of power supply the customer had.
Once you go over a 5kw inverter, you cannot feedback into the grid. There can be some exceptions to this rule through special approval in NSW, but if you're in QLD there won't be.
You can only use the extra power of a second inverter, or an 8kw for example if you are using more than 5kws at one time! This is a lot of power, and for most people, they will very rarely, if ever actually utilize the extra power.
So please listen if we tell you that you don't need to go as big as you've been told, we're not going to sell you something you can't use. Technically, it's actually illegal as it is goods not fit for purpose!