The recently expanded San Vicente Reservoir could become a backup source of electric power under a plan being considered by the San Diego County Water Authority and the City of San Diego. The agency and city, which owns the reservoir, are studying a 500-megawatt pumped-storage project — big enough to power a quarter million homes during periods of high electrical demand. An official request was issued Wednesday to utilities, developers and investors potentially interested in the project with response expected by Feb. 15.