Artificial roughness applied to the Solar Air Heater (SAH)’s absorber plate is a popular technique to increase its total-thermal efficiency (η_(t-th)). In this paper, the influence of geometrical parameters of V-down ribs attached below the corrugated absorbing plate of SAH on the η_(t-th) has been examined. The impacts of key roughness parameters, including relative pitch p⁄e (6-12), relative height e⁄D (0.019-0.043), angle of attack α (30°-75°), and Re (1000-20000), are examined under real weather conditions. The SAH η_(t-th) roughened by V-down ribs is predicted using an in-house developed conjugate heat transfer numerical model. It has been shown that the maximum SAH η_(t-th) is 78.8%, predicted under steady-state conditions of Re = 20000, solar irradiance G =1000 W/m2, p/e = 8, e/D = 0.043, and angle =60. That is 15.7% more efficient than that of the default smooth surface. Under real weather condition, the η_(t-th) of the roughened SAH with single and double glass covers are 17.7% and 20.1%, respectively, higher than those of smooth SAH.