Its half a barn roof, I believe its was around $15k installed.

Main issue I have seen with the law in FL for them is that they are net metered but the power bill can't go negative. So if they produce more than they use the power company doesn't have to pay them for it, also they always get a service charge no matter what. So their power bill ends up being about $20 a month no matter what which is just the service charge.

That's how net metering works for me in Oregon, too - I earn credits by over-producing all summer, and then use some of them up until Jan 1, as they won't roll over year-to-year. As a result I gave power away for free to the grid for several years, but now I have a plug-in hybrid to soak up the surplus.

In Australia 5kw is extremely common (20 250 watt panels) - out of pocket it's ~$5000 AUD.

Now that 280 watt panels are becoming common, up to 7kw is a "standard" install.

But you need three phase to go beyond 5kw if I understand correctly

I think you have to get permission to ensure that not too many people in your immediate area have over 5kw, otherwise the grid in that area might not be able to handle so much being pushed back in without upgrades.

I don't know about three phase.