1. It is very unlikely you'd ever be able to borrow money from a home loan at an interest rate BELOW even the best high interest deposit account. Banks are very good at arranging that part of your 'financial management'.
2. The estimate in 2011 was that power bills would double by 2015-16 -
I feel this is a bit alarmist but increases of up to 50% are on the cards so you could be paying $450 - $600 per Qtr. by 2015-16 or $1,800 - $2,400 per annum. This would reduce your payback period by several years.
3. Prices for solar systems currently are probably based on an exchange rate above $1.05 and may increase slightly given we are now trading at $0.98.
4. The expected lifetime for solar panels is estimated at 20 - 25 years (at at least 80 - 85% output) and inverters at about 10 years + depending on build quality. So you get 10+ years where it's free.
5. Solar PV systems will increasingly be valued by prospective home buyers which will provide a Return on Investment down the track. There are also moves afoot to grade houses for energy efficiency which are now already in place in Europe and will impact both sales and rental prices.
All-in-all I think investing in a solar PV system now is a good idea and yes they will possibly get cheaper but given the carnage that is happening to solar PV manufacturers worldwide with wide spread bankruptcies they may not continue to drop as precipitously as they have been.