Thermal performance of solar air heater does not take into account energy loss due to friction for propelling air through the duct. Therefore, it is necessary to evaluate thermohydraulic performance in order to investigate simultaneous effect of thermal and hydraulic characteristics on performance of solar air heater. In the present paper thermal and thermohydraulic performance of smooth as well as roughened solar air heater has been investigated with the help of a mathematical model. Absorber plate of solar air heater has been roughened with the formation of protrusions. Optimum value of each roughness geometry parameter has been obtained on the basis of thermal and effective efficiency of roughened solar air heater. Design plots have also been prepared in order to facilitate the designer for designing such type of roughened solar air heater within the investigated range of system and operating parameters. (C) 2012 Elsevier Ltd. All rights reserved.