WASHINGTON (Reuters) - Conservative U.S. Supreme Court justices on Wednesday expressed doubt about whether the Obama administration had the authority to issue a regulation aimed at encouraging efficiency in the electricity market by having electrical grid operators pay users to reduce consumption at peak times. The court heard oral arguments in an appeal filed by the U.S. Federal Energy Regulatory Commission seeking to reverse a May 2014 ruling by the U.S. Court of Appeals for the District of Columbia Circuit that threw out the rule. The case pits the government's energy regulator against power companies that are fighting a regulation...