U.S. Dept. of Energy, Apr. 3, 2017 by Jason Hartke.
https://energy.gov/eere/articles/play-ball-american-baseball-stadiums-round-bases-energy-efficiency

"As Americans gear up to “root, root, root for the home team” on Major League Baseball’s (MLB’s) Opening Day, they might be too focused on the games to notice that their stadium is more efficient than it used to be. Professional ballparks average 1 million square feet and together host more than 73 million Americans each year – which means there are a lot of opportunities for improvements that enhance visitor experience, save stadium owners and operators money, and reduce energy and water use."