This study investigates the effects of orientation of photovoltaic surface and proposes the optimum tilt angle for a photovoltaic array oriented due south in three cities in Nigeria (Abuja, Benin City and Katsina). Three optimization methods (monthly based, seasonal based and annual based) are implemented. The inclination of the surface is assumed to be varying from 0° to 90° with an increment of 1°, and the total global solar radiation on the tilted surface is estimated using the Hay-Davis-Klucher-Reindl (HDKR) Model. Analysis indicates that the photovoltaic (PV) surface positioned at monthly optimized tilt angles will generate an increase exceeding 10% of its annual total irradiance.