In the designing of solar photovoltaic (PV) systems, which are directly coupled to batteries, it is of great importance to consider voltage-operating points of PV modules and batteries. Usually, batteries operating points are supposed to be near to the PV maximum power points to maximize the PV output. PV output voltage is affected by incident radiation, air temperature, shading, module orientation, PV technology; tilt angle and number of serial connected PV cells. This paper presents a PV model, which takes into account the number of PV cells connected in series, incident solar radiation and temperature as main factors affecting PV systems that are directly coupled to the batteries. The developed model was then validated by conducting some laboratory tests. In this study, an amorphous silicon thin film PV module was used. Electrical load data collected in Dodoma, Tanzania and lead acid battery with 24 V were also used in the designing and simulation of PV systems. Optimization and the laboratory test results have revealed how good the optimization tool is. The tool can select the optimal number of PV cells required for the given system. The selected optimal number of PV cells in series always matches with the battery operating voltage, which helps in the reduction of losses that usually occur in direct-coupled systems, hence maximizing PV output. The results presented in this paper are based on solar home system simulated in Dodoma, Tanzania; however, the model can be adapted to any other region and any type of PV module by changing input data like solar radiation, air temperature, longitude, latitude, load profile and STC parameters of the PV module respectively.