Shawn Horcoff and Tyler Seguin also had man-advantage goals, and Colton Sceviour also scored as Dallas completed its season-high, five-game road trip with three straight victories. The Stars won four of five overall.
The three power-play goals are the second most the Stars have scored in a game this season. They netted four in a 5-2 victory over Winnipeg on Jan. 31.
Kari Lehtonen made 18 saves and has won six consecutive games versus the Capitals. The Stars have beaten Washington seven straight times.
Curtis Glencross and Evgeny Kuznetsov scored for the Capitals. Braden Holtby stopped 18 of 22 shots before being replaced after Fiddler made it 4-1 in the second period. Justin Peters stopped all nine shots he faced in relief.