Interesting.
Can I see your payback calculation and assumptions on solar electricity generation?
in NSW, grid-connected solar systems are hooked into a GROSS meter system. That means that ALL solar produced electricity is put onto the grid, and you get a credit for it.
Solar irradiation is a pretty consistent number... it is worked out in "peak sun hours".
In Sydney, that is around 4 hours per day, averaged throughout the year. (in winter its about 2.5 hours/day, and 6hours/day in summer)
Thus, working out how much credit you will make per year, on average, is pretty easy.
Size of solar panel array x Avg. peak sun hours x 365days x 0.66c x 0.9(inverter efficiency, panel coefficient losses, etc...)
Thus, with skater's 1.5kW, she should make around $1300/year in credit.
Of course there are other factors such as shading, non-optimal orientation, etc.... but $1200-1300/year is a fair estimate.