Isn't 70% or more of the Earth's surface covered in water and thus, quite difficult to build a terrestrial structure upon?

Asking for a friend.

Total land area needed to power the world on solar alone: http://i.imgur.com/Jg31dfw.png

Source: http://www.businessinsider.com/elon-musk-solar-panels-to-pow...

EDIT: Solar is a better investment than the S&P500. If US corporations were looking for some place to invest those trillions of dollars they won't repatriate [1], seems like renewables would be a perfect fit.

"Eight of the biggest U.S. technology companies added a combined $69 billion to their stockpiled offshore profits over the past year, even as some corporations in other industries felt pressure to bring cash back home.

Microsoft Corp., Apple Inc., Google Inc. and five other tech firms now account for more than a fifth of the $2.10 trillion in profits that U.S. companies are holding overseas, according to a Bloomberg News review of the securities filings of 304 corporations. The total amount held outside the U.S. by the companies was up 8 percent from the previous year, though 58 companies reported smaller stockpiles."

[1] http://www.bloomberg.com/news/articles/2015-03-04/u-s-compan...

Good thing an hour and a half is well less than 30% of the day, then.

McKay uses something very close to this (as you said) theoretical value as the starting point of its calculations for how much can effectively be extracted.