Present work reports the findings of experimental investigation performed outdoors in real time conditions on a novel roughness geometry frustrum roughened solar air heater. The test rig was designed and fabricated at rooftop of Mechanical Engineering Department, MANIT Bhopal, India. The geometrical specification of frustrum geometry machined to the absorber was varied and tested. Two new roughness parameters to determine its impact on thermal performance is introduced namely relative diametral ratio (d1/d2) and roughness height to base diameter ratio (e/d1) and varied respectively as 1.33–3.00 and 0.16–0.75. Relative roughness pitch (p/e) and height (e/Dh) is respectively varied as 8–14 and 0.013–0.054. Smooth and roughened absorbers are tested under varying range of geometrical and flow parameters (Reynolds number 3500–12500). The investigation revealed that thermal performance is strongly dependent upon flow Reynolds number and temperature rise parameter. Thermal efficiency is expressed in terms of collector efficiency factor and heat removal factor varying respectively in range of 0.76–0.81 and 0.60–0.75. Thermal efficiency varied in range of 42–79 % for different mass flow rates and roughness parameters. The maximum thermal efficiency obtained is 79 % at p/e = 12, e/Dh = 0.040, e/d1 = 0.75, d1/d2 = 3 for mass flow rate of 0.0486 kg/s.