Regulatory subunit of a potassium efflux system that confers protection against electrophiles. Required for full activity of KefC. Shows redox enzymatic activity, but this enzymatic activity is not required for activation of KefC. a quinone + H(+) + NADH = a quinol + NAD(+) a quinone + H(+) + NADPH = a quinol + NADP(+) Homodimer. Interacts with KefC. Belongs to the NAD(P)H dehydrogenase (quinone) family. KefF subfamily.