Once you have drawn a projected solar array on top of the satellite imagery from Google Maps you get a rough estimate on performance and payback of your system based on the drawn area of the array, slope of the roof, location and weather information from the National Renewable Energy Labs.

To complete the full financial analysis your project should be in California as for now utility rate information is only available there (other states will be added soon).

See a tutorial video below and check out the RoofRay site

Via earth2tech