Unit of capacitance. A farad is defined as the amount of capacitance for which a potential difference of one volt results in a static charge of one coulomb. It has the base SI representation of s4 • A2 • m-2 • kg-1. Since an ampere is the rate of electrical flow (current) of one coulomb per second, an alternate definition is that a farad is the amount of capacitance that requires one second for a one ampere flow of charge to change the voltage by one volt.