The history of solar energy is full of predictions or "possibilities" that were not realized and appear in retrospect to have been too-wishful. Here's an example which I mentioned in The Solar House. In 1952, the Paley Commission completed a five-volume report entitled Resources for Freedom, which predicted future oil shortages, warned against dependence on imports, and recommended a national energy agency. It included a section called “The Possibilities of Solar Energy”, written by Palmer Putnam:
"Of new dwelling units to be built in the United States by 1975 ... functionally designed for solar heating ... the maximum plausible market may be more than 13 million installations ... supporting about 10 percent of the national energy system."
In fact, by 1975, solar energy was such an insignificant fraction of the national energy supply that it could not be quantified (pdf source), although this type of analysis is subject to The Clothesline Paradox.