The National Renewable Energy Laboratory has released The Open PV Mapping Project showing the progress of solar installations on private roofs from 2000 to (currently) now, beginning with a flurry of activity in California, then moving to Wyoming, of all places, and WIsconsin. Gradually the states turn yellow and then orange over time as they add more solar power.
They are soliciting your input, if you have installed solar. It is a community driven project and only as accurate as the data entered in a collaborative effort between government, industry, and the public. As they point out, currently, it is only estimates.
For example, many of the states have no contributors as of yet. The project will also help gauge the progress toward DOE’s goal of reaching PV grid parity by 2015.
Source: DOE EERE