Αs iTech pointed out, kratos isn't the word you want here, since it means "physical strength," "dominance," "domain." You want the more general term for "power," "ability," "potential," and that's dynamis. So "Knowledge is power" = Gnosis dynamis, or ἠ γνῶσις δύναμις, = (in caps) Η ΓΝΩΣΙΣ ΔΥΝΑΜΙΣ.