Supercapacitors can improve energy storage performances and capabilities, in particular for applications requiring high specific capacitance and high specific power. Unfortunately, supercapacitors are characterized by several non-ideal and complex behaviors that must be modeled and taken into account. One of the most relevant issue consists in the definition of the equivalent capacitance and then to the energy that can be potentially stored and practically used. This paper discusses the possible capacitance definitions and measurement methods, including the possible approaches to model the capacitance and its non-idealities.