For the time ever, Xcel Energy locked the smart thermostats of thousands of Colorado customers on Tuesday because of what it called an energy emergency. Xcel started a program six years ago, called the AC Rewards program, in which customers get rebates in exchange for allowing Xcel to adjust their thermostat on the hottest summer days to ease strain on the electrical grid. Participating customers need to sign up for the program, and about 22,200 have done that in Colorado. Tuesday was the first time that Xcel locked those thermostats, a company spokesperson said. The grid experienced an energy emergency...