A low fare from Berlin to San Francisco from $370 to $392 is available with SkyTeam or Oneworld routes for departures from mid-August into May 2018.
Fares are low $400s over December holidays.
As a Central Coast California local, January is a great month to visit this area of California when gray whale migration to Mexico happens.
I generally see 100 or more gray whales in late December and January watching from the Monterey County, California coastline. The weather is often gorgeous and some days can even be warm (25+ C) in January in between Pacific winter storms.
American Airlines ticket
Berlin TXL – Helsinki HEL – San Francisco SFO – London LHR – Berlin TXL
Air France Ticket
Berlin TXL – Paris CDG – San Francisco SFO – Paris CDG – Berlin TXL
United Airlines ticket (Austrian Airlines outbound)
Berlin TXL – Vienna VIE – Washington IAD – San Francisco SFO – Newark EWR – Berlin TXL