The most important, and most accessible design parameter of a series integrated thin-him photovoltaic module is the cell length l(c). Its optimal value l(copt) is a trade off between area loss, and series and shunt losses. If the design is performed at standard reporting conditions (i.e. constant one sun illumination), too low a value of l(copt) results. When the design is done for a realistic histogram of the incoming sun power P-in, a larger value of l(copt) follows, and an efficiency gain of a few tenths of a percent (absolute) is obtained. A climate model, enabling to calculate the histogram of P-in for given cloud conditions is presented here. A model to calculate the module parameters was presented before (Burgelman and Niemegeers, Sol. Energy Mater. Sol. Cells, June 1998). Both models are then applied together. As a result, the optimal Value l(copt) and the efficiency gain are calculated as a function of the location (geographical latitude) of the panel, both in clear sky and in clouded conditions.