Utility owner Xcel Energy Inc. has announced it is to buy two power plants from Calpine Corp. for $739 million. The purchase includes the 621-megawatt Rocky Mountain Energy Center near Hudson, Colorado, and the 310-megawatt Blue Spruce Energy Center near Aurora, Colorado. The plantsÔÇÖ combined capacity can power about 700,000 homes. The Rocky Mountain and Blue Spruce plants, both fueled by natural gas, sell power to Minneapolis-based XcelÔÇÖs Public Service Co.