The history of solar energy is full of predictions that were not realized and appear in retrospect to have been too optimistic. This is not a retroactive critique, but perhaps it's a reminder to make forecasts with modesty.
In 1979, the U.S. Department of Energy studied the future prevalence of passive solar houses and light commercial buildings. They expected that in the year 2000, 41% of new buildings would be passive solar. That assumed no incentives. With tax credits, the projection rose to 48%. (source)
Also in 1979, a report from the Harvard Business School concluded:
"Solar energy, the use of thermal (heating and cooling) applications, fuels from biomass, and solar electric methods, could produce one-fifth to one-fourth of America's energy needs by the year 2000."
Source: Energy Future Report, Robert Stobaugh and Daniel Yergin, eds., (New York: Random House), 1979. As summarized by William A. Mogel (source; pdf)