Sonnen, a German residential energy storage developer, has opened its U.S. headquarters in Los Angeles in its bid to become a big player in the home battery market in America, according to Greentech Media. It counts Tesla as its chief rival in the residential solar energy storage market.

While Tesla has partnered with SolarCity to get homeowners on board with its product, Sonnen announced Friday that it is teaming up with SolarWorld, the largest crystalline-silicon solar manufacturer in the United States, and PetersenDean, a Fremont roofing and solar company, to offer a complete solar-plus-battery package for the residential market. Partnerships are a key element of Sonnen’s expansion plans.

Sonnen’s residential battery system is described as an integrated system that incorporates a Sony Fortelion lithium-ion phosphate battery, an inverter, and a control system that can be accessed via computer or smart phone, or through a user interface on the battery.

The grand opening of Sonnen’s headquarters in Los Angeles, meanwhile, cements the company’s presence in America, building on the launch of its manufacturing plant in San Jose, and its research and development hub in Atlanta