Hi Timmy
A 10kW system will make about 40kWh a day on average throughout the year in Ballarat, although of course it will produce more in summer and less in winter, which is the opposite of when you use it unfortunately. See our Solar Generation Victoria page for more details around that.
A 6.6kW system would possibly provide a faster payback, because you’ll be using most of your solar year round, which gives a faster payback usually than exporting it. I think the Vic solar rebate maxes out after 5kW or something as well doesn’t it? I’d personally probably look at a 6.6kW system, even if you didn’t reduce your usage. However, reducing your usage as discussed would of course be best way to save money (and reduce emissions if that’s a consideration) - I know it gets cold in Ballarat though, I’m sure it’s hard to keep the place warm.
That aside, all three of your quotes use pretty decent equipment. I’d be happy with any of them really. SunPower and Fronius is a standout, but also significantly dearer, so hard to way up if it’s worth the extra cost.
As I always say, most important thing is to get a good installer. Have you checked out their reviews? See this chapter in our Buying Solar guide if you haven’t already.
https://forum.cleanenergyreviews.info/t/ch-2-who-should-i-buy-solar-from/155
Cheers
Marty