This paper presents a new index, called the "evaporative capacity", for rating the performance of the solar air heater in a solar drier consisting of solar air heater and a drying chamber in series. The proposed index complements the widely-used "collector efficiency" as a performance indicator of the solar collector, by taking into account the specific use that is to be made with the heated air. Presented is a detailed method for calculating the evaporative capacity, and a comparison of this new index with the thermal efficiency index, demonstrating its superiority. General charts for a rapid determination of the evaporative capacity are presented, and some possible applications of these charts are described.