For a given solar panel rating, the dollars per year that it produces depends on the annual sun and the local electricity price.
In Detroit, a 380 W panel produces $80 per year of electricity.
We sell a 380 W solar appliance for $465, shipped ($1.22/W). After the 30% tax credit, your outlay is then $326.
At $80 per year, it pays back $320 in 4 years.
Our 760 W double kit pays back a bit faster than that. ($465 + $380) The cost of the double kit is $845, shipped. $845 becomes $592 after the 30% tax credit. $592 / $160/yr = 3.7 years.
For more information see "Savings" in FAQs.
Check Your States Electricity Cost.
Use NRELS PVWATTS website to calculate your payback time.
Google Project SunRoof