This paper describes an optical-irradiance model devised to analyze the performance and geometrical optimization of a single slope solar still. The three-dimensional backward ray-tracing technique, along with Latin Hypercube Sampling (LHS) was used to statistically generate the sampled incoming rays. Validation was conducted in four geometrical scenarios using ray-tracing with a single ray. Results were then compared to the output of the 3D modeling program. The sampled rays were then simulated to obtain complete collection of irradiance in these scenarios and the optical efficiency was evaluated. The model was then used to investigate the effects of changing the aspect ratio and the cover angles on the optical and irradiance performance of six different still designs. Later, the model was used to develop powerful tools with the following applications: (i) selecting the optimum aspect ratio and cover angle so the desired optical performance can be obtained at a given time of day and (ii) evaluating the day-round performance of a still design based on the combination of the ranges of aspect ratio and cover angle. They are explained via a case study conducted in Auckland (New Zealand).