This paper introduces an irradiance loss factor that quantifies the relationship between irradiance, tilt angle and power output of a soiled panel with the soil particle size composition. Artificial soiling experiments were performed using four soil samples at irradiance levels between 200 and 1200 W/m(2) at 18 tilt angles. Biharmonic interpolation was used to develop power contours in terms of irradiance and tilt angle from experimentally obtained data. These contours were compared with ideal ones of a clean panel to observe deviation in the nature of contours for a soiled panel. A correction factor in terms of particle size composition (as a coefficient to tilt angle) was proposed to calculate power output of a tilted soiled panel. The angular loss on a panel with soil sample containing 150 mu m particle size in abundance was observed to be 22% and for sample containing 75 mu m particles in majority, the loss is 24%. Presence of 300 mu m particle size in abundance causes a 23.7% loss, while 52% angular loss was observed for soil with highest composition of less than 75 mu m particle size. (C) 2016 Elsevier Ltd. All rights reserved.