In this paper a general method is presented for the direct determination of the optimum tilt angle of solar photovoltaic arrays, taking into account the latitude of the place, the local climatological and insolation conditions and the orientation of the photovoltaic arrays, independently of the particular application. The optimization method is based on the minimization of the relative error (with respect to the annual mean insolation on a tilted surface for each tilt angle of a given orientation) in the average monthly insolation on the tilted surface for those months for which this insolation is less than the annual mean. In this process the optimum tilt angle is selected so that it leads to the maximum degree of uniformity of insolation throughout the year and hence to the photovoltaic system with the smallest installed peak power and battery capacity required for any given application. The method has been applied to obtain optimum tilt angles for south-facing as well as off-south-facing modules for a number of different places in Greece. The optimum tilt angles of photovoltaic arrays for the azimuthal angles in the range from 0 ° to -+60 ° studied were found always to be greater than the latitude of the site by about 40% -60%, and to be constant for azimuthal angles of up to -+30 °. The array optimum tilt angle was found to be a linear function of the clearness factor at the particular location. The values of the optimum tilt angles obtained directly using this method are in excellent agreement with those obtained indirectly using the method of selecting the "optimum" photovoltaic system.