President Donald Trump decided Jan. 22 to slap tariffs on imported solar panels, the biggest blow he’s dealt to the renewable energy industry yet.

The U.S. will impose duties of as much as 30 percent on solar equipment made abroad, a move that threatens to handicap a $28 billion industry that relies on parts made abroad for 80 percent of its supply.

Just the mere threat of tariffs shook solar developers in recent months, with some...