The Sacramento County Board of Supervisors on Tuesday agreed to partially refund utility and property taxes if the company makes investments and reaches employment targets at its proposed 650,000-square-foot plant at the former Air Force base. The Hayward-based company plans to start construction on the local plant this summer, and hire 50 employees by the end of the year. The county expects OptiSolar to generate about $46 million in new tax revenue with the rebates. The Sacramento Area Commerce and Trade Organization, also known as SACTO, estimates the plant will stimulate an additional 1,700 jobs and have an economic ripple effect of more than $200 million to support industries in the region every year.