Win Probability Added is kind of a convoluted stat, but an interesting one. Click here for a primer. It attempts to measure how pitchers and hitters effect their team's chance of winning based on the outcome of every plate appearance (defense is not part of the stat). That means it is context specific; as the Fangraphs primer linked above says, a walk-off homer will be worth more than a homer in a blowout.
If Jason Kendall's "bat control," "productive outs," "sacrifices," and OHMYGODHE'SSOGRITTY actually contributed something to the team, this is the stat where it would show up. You can see how that's going for him. Good thing his defense is so incredible.