The Nationals remained perfect in five home games this season, getting a seven-inning performance from Jordan Zimmerman and a mammoth homer from Bryce Harper.
Harper hit a shot into the second deck in right field in the fourth to create a 1-1 tie and Espinosa singled home Ian Desmond later in the inning to put the Nationals in front for good.
Ryan Zimmerman singled in a Washington run in the fifth and the Nationals added two in the sixth with a double by Espinosa driving home one of the scores.
Zimmerman (2-0) allowed seven hits and struck out four. Rafael Soriano worked the ninth for his fourth save.
The loss went to Gavin Floyd (0-2), who allowed five runs on nine hits in 5 1/3. He struck out seven.