You don't use much power so you might be ok with that.
Average household uses about 18.
How much north facing roof do you have though? Enough for all the panels? As you lose about 10% if you they face east or west. Do you have any issues with shade (large trees?, tall neighbour houses?) , if you do you might lose some power generation there. Also, if you have a flat (or fairly flat) roof, make sure you get tilt frames as you lose about %10 there as well, plus the panels get dirty a lot quicker due to not having good rain runoff.
Not exactly sure where in WA you are - but you could look on this websitehttp://www.pvoutput.org
and search on your postcode to find others in your area to see how much they generate with their systems.
In Perth - this person: http://www.pvoutput.org/list.jsp?id=725&sid=563
has a 1.5kW system and they generate an average of 6.5-7.5kWh per day.
You'd need to check with your power company - but I don't think you get much for feeding power back into the grid there - so you probably want something mainly to cover your daytime use, so probably somewhere in the vicinity of 1.5-2kW would be fine for you (you may need bigger if you have any of the issues I mentioned earlier). Then just try and use most of your generated electricity during the day (as it gets used in your house before being exported to the grid), and not use much power at night.
If you were in my state I'd say get as big as you can afford/fit on your roof as we get paid the same amount for what we export to the grid as what we import - so it doesn't matter what time of day we use the power as we get a decent amount for it exported.
Aurora or SMA inverter with either REC or Suntech panels are supposed to be really good. However the cheaper systems will will still produce nearly as much - they just might not last as long.
Get some more quotes - the price difference between the good stuff & the cheap stuff is not supposed to be much (maybe a couple of hundred) and if they last an extra 10 years that might be worth it.
I would not get a bigger inverter and hope to add panels later, unless you are planning to do that within a year or so. First of all, the new panels would need to match the specs of the existing panels, which may be difficult in a few years time. Second would be that you need to wire the panels up a certain way so they produce the optimum amount of voltage for the inverter - adding extra panels may mean rewiring the whole system to get that. You are better of getting something that will generate enough right away (if thats what you want it to do), and you can always add another complete system later (without having any of the matching issues).
This post has been edited by laridae: 04/05/2012, 10:33 PM