The California Energy Commission (CEC) through the Public Interest Energy Research Program (PIER) commissioned a report from Navigant Consulting that has quite a bit of useful data (as of September 2007) for solar market sizing. The report is titled "California Rooftop Photovoltaic (PV) Resource Assessment and Growth Potential by County, and can be downloaded here.
The report focuses on the potential for PV installations by county for the state of California, and takes into account such factors as roof tilt, shading, declining installations costs, tax credits, etc. in order to arrive at a total possible installed generating capacity.
One thing that really jumps out is that Navigant estimates a max of 500MW in installed panels by 2016 without subsidies, but that the use of subsidies will increase the total installed base to 800MW.
Then Navigant makes another assumption that "new business models" such as power purchase agreements (PPA's) come into play. Using PPA's plus all available subsidies, the total installed market jumps to 1,700 MW by 2016. So in effect the PPA's could have more impact than the subsidy.
Finally, the goal expressed by the state of California is to reach 3,000 MW. The report states that only way to reach this goal is to assume that technological breakthroughs (like thin film or advanced crystalline silicon) lead to a large price reductions in installed cost.
A blog on using the power of Disruptive Business Models to build successful businesses...and other stuff. by Joe Agliozzo