Reuters: First Solar Inc said on Monday it would pay rival OptiSolar $400 million in stock for its pipeline of solar projects, including a major installation for California utility PG&E Corp and other nascent deals that will rapidly expand the company’s presence in the U.S. utility market. The deal, which First Solar Chief Executive Mike Ahearn called a “watershed acquisition” for the company, includes 1.3 gigawatts (GW) of solar development projects being negotiated with Western U.S. …