My guess is we can build several LEO demonstration SSP, of increasing power up to ten megawatts by 2016 at a cost of several billion dollars. After that it may not be practical to build more power in GEO orbit, but we won't know unless we try. Also some needed technology for large scale likely will not develop unless we proceed with enthusiasm.
Other alternative energy than fusion and SSP are likely impractical, except as small scale or medium scale. Failure to reduce our dependence on Arab oil will likely prove far more costly. Let's start writing SSP reasarch contracts. See http://spacesolarpower.wordpress.com Neil