Use the noun potency when you're talking about the strength of something. You could refer to the potency of a drug or the potency of a political leader.
Potency is an official measurement in pharmacology, or the science of prescribing drugs. It's also useful for describing a person's power or authority over others. The Latin root word is potentia, or "power," and its origin goes all the way back to the Sanskrit word for "master or husband," patih. The word potency has historically been associated with men, but it's fine to use it to describe a woman's power too.