Solar Power Stats - Payback Times

I asked my son to calculate the approximate savings in $$ terms since installing the 1kW Photovoltaic system in Dec 2009 (Feed-in meter installed Jan 2010) to estimate how much of the purchase price of the system we have recouped. (Payback!). So here goes.

1. Total Generation (up to 24/3/2012) = 2920 kWh
2. Total Feed-In (24/03/2012) = 848.8 kWh
3. Total Generation minus Total Feed-in = 2071.2 kWh

Taking all the changes in electricity charges into account and estimating a 5/7 peak and 2/7 off-peak tariff weighting (weekends are charged at off-peak rates), as well as the time period we didn't receive the  premium feed-in tariff of 66c:

We have received a benefit of $869.32 of "free" electricity.
Given we had a total cost (minus government rebates) of $1895.00 to purchase the system we should be able to totally pay-back the system in 4.99 years.
Of course, since the government provided the rebate, the payback period without it would have been massive at  26.34 years (Assuming no more changes in electricity prices, a patently false assumption)
However, since the installation of the system in Dec 2009 the price of silicon has plummeted and the cost of equivalent systems has also fallen (supply and demand stuff). The $1800 we received by the Federal Government GFC package has been well spent indeed!
So far experiments with photovoltaics have shown good systems have a small fall-off in efficiency after 20 years, so I expect, barring very vicious hailstorms, and inverter failures, to have those panels gently humming away for a long period.
I'm not up-to-date with cost of new systems and their rebates but there still seems to be plenty of people installing systems, judging by the proliferation on houses around me. So I expect many will have calculated a reasonable pay-back period as well as experiencing the glow of doing something good for both environment and jobs in our local economy.
PS - My son's maths is better than mine!!