U.S. trade officials slapped modest tariffs on imports of Chinese solar panels, giving a partial victory to solar-equipment manufacturers in the U.S. but stopping short of harsh duties that could spark a trade war.

Responding to a complaint by the U.S. unit of Germany's SolarWorld AG and six other firms who complained about competition from Chinese rivals, the Commerce Department announced preliminary duties of between 2.9% and 4.73% on Chinese solar cells and solar panels.

The...