The states, up from eight last year, plan to darken governors' mansions and other buildings and monuments as part of Earth Hour, the World Wildlife Fund's call to action on climate change.
"It reflects the conviction of people around the world that climate change is real, and we need to do something about it," said Carter Roberts, head of the fund.
Earth Hour has spread globally to 87 countries since starting in Australia in 2007, USA Today reported Friday. This year, 2,200 businesses, including Walgreens, and millions of homes are expected to shut off all but essential lights at 8:30 p.m. local time, Roberts said.
Monuments taking part were to include New York's Empire State Building, San Francisco's Golden Gate Bridge, Seattle's Space Needle, the Las Vegas Strip, Niagara Falls and the Willis Tower, formerly the Sears Tower, in Chicago.