Despite several decades of research in the field of photovoltaic (PV) systems, shading tolerance has still not been properly addressed. PV modules are influenced by shading concerning many factors, such as number and configuration of cells in the module, electrical and thermal characteristics of the cells, number and type of bypass circuits, electrical characteristics of bypass elements, and shading profile features. Along with the random nature of shading profile over the lifetime of a PV system, it is difficult to choose the best module for a location which is most of the time sunny, partly cloudy, or cloudy. This paper suggests a measurable parameter, the so-called shading tolerability (ST), to classify PV modules regarding the ability to oppose shading effects. Based on mathematical and probability analysis, the ST parameter is extracted and then measured using a large area steady state solar simulator. Finally, the results of on-field experiments are presented as a proof for the shading quantification method and its significant contribution to performance ratio improvement.