In this paper, a new approach has been made to obtain the characteristic equation of a double slope passive solar still (DSPSS) based on experimental observations. The performance of DSPSS has been analyzed for the composite climatic condition of New Delhi, India. To obtain the characteristic equations under quasi-steady state condition, regression curves have been plotted for instantaneous gain and loss efficiencies with respect to a non-dimensional representative factor a(T-w-T-a)/I(t))/((T-w-T-a)/I(t)(max)) of climatic and operational parameters together. From the analysis, it has been concluded that non-linear characteristic curves are more accurate for analyzing the performance, thermal testing and further design modification depending upon various parameters associated with design, climatic and operational conditions. (c) 2010 Elsevier B.V. All rights reserved.