Measuring the I–V characteristics is of high importance since it can be considered as a quality and performance certificate for each PV generator. The most precise and inexpensive measuring method is represented in capacitor charging by the PV generator. Using the equivalent circuit of the PV generator with a capacitor as load and applying transient analysis on the circuit, we obtain the capacitor charging voltage and current as a function of time, as well as their differentials as a function of short circuit current and capacitor size. The derived equations facilitate the calculation of proper capacitance size for measuring the I–V characteristics, and considers the acquisition speed of the measuring system as demonstrated through two measurement samples in this paper. The capacitor size is directly and indirectly proportional to the short circuit current and open circuit voltage of the PV generator, respectively. Accordingly, the paper presents a capacitance calculation chart, which enables selecting the correct capacitance for measuring the I–V characteristics by a computerized data acquisition system.