Tesla has built a battery farm for a California utility company. The facility contains enough batteries to run about 1,000 Tesla cars or to power 15,000 homes for four hours. The project marks an important point in Tesla’s strategy to expand beyond the electric car business. Developing battery packs is a core expertise for the company, which is designing packs for homes, businesses, and utilities. It markets them partly as a way to store solar electricity for use after sundown, a pitch that works well for states with a booming solar energy market such as California.