Here is an interesting item of news from the July Solar News:
ROOF MOUNTED PHOTOVOLTAIC SYSTEMS
The price of grid connected roof mounted PV systems has recently decreased significantly and they now represent a very good return on investment. For example a 1.5kWp system can be installed for approximately $3000 plus approx $400 for an import/export meter. This type of system if mounted in a good location should generate approximately 2200kWh/yr. If power usage is well managed then at least half of this can be exported which with the feed in tariff of $0.50/kWh should generate revenue of approx $550/yr.
The remainder will offset electricity use in the house at approx $0.20/kWh saving another $240. This gives an annual return of approx $750 to $800 and a simple payback time of only 4 to 5 yrs. Even if money has to be borrowed to install the system this still gives a good economic return.