Thomas Chabot scored Ottawa's third power-play goal at 5:18 of the third period, beating Kahkonen five-hole with a one-timer from the right circle to make it 4-2.
"I don't know exactly what happened, but we killed ourselves in the second and third because we took way too many penalties, especially in the [offensive] zone," Hertl said. "It just can't happen."
San Jose entered Saturday first in the NHL on the penalty kill at 91.6 percent (65-for-71) and had not given up more than one power-play goal in a game this season.
"Well, I didn't win any face-offs," Couture said. "So when you start without the puck, you give the other team a power play, that's not a recipe for success. It starts there. And then some of their set [plays], we knew what they were going to do, and we just didn't cover properly."
Giroux extended the lead to 5-2 at 15:45 with a slap shot on a breakaway shortly after the Senators killed off a 4-on-3 power play for 1:26.
"In between periods, I told [Shane] Pinto I was gonna score on a slap shot," Giroux said. "So I didn't really have a choice."
NOTE: Chabot's goal was his 200th NHL point (46 goals, 154 assists) in his 332nd game. Among defensemen in Senators history, only Karlsson (272) has reached the milestone in fewer games.