Mike Green and Tomas Fleiscshmann each scored a pair of goals in Washington's fifth consecutive win.
The loss spoiled the debut of Flyers Coach Peter Laviolette, who took over Friday when John Stevens was let go.
Ray Emery started in goal for Philadelphia and took the loss after allowing five goals on 17 shots in 31-plus minutes. Brian Boucher was touched for three goals on 13 shots.
The Capitals scored seven of their goals in the first 40 minutes of the game, including two goals during a 9-minute man advantage in the first period that resulted from a 10-minute misconduct and a game misconduct handed to Philly enforcer Dan Carcillo. Carcillo had decked Washington's Matt Bradley with one punch.
The long power play resulted in a 4-1 lead for Washington that grew to 7-1 at the end of two periods.
"We put ourselves in a bad position from the start and it all went downhill," said Scott Hartnell, who scored for the Flyers along with Mike Pyorala. "There are no excuses when you lose 8-2."