Every second, the sun bathes Earth with 172,500 terawatts of energy. (A terawatt—a trillion watts—is the biggest energy unit in common use.) About a third of this prodigious flow is promptly reflected into space, mainly by clouds. The leftover—roughly 113,000 terawatts, depending on cloud cover—is available for capture. All human enterprises together now use a bit less than 18 terawatts.
