The performance of a photovoltaic (PV) module is influenced by its tilt angle with the horizontal, due to the tilt angle change the amount of climate condition (solar irradiance and temperature) reaching the surface of the PV module. The optimum tilt angle is needed to get the maximum solar irradiance and the minimum temperature along day reaching the surface of the PV module. This paper presents a calculation of the tilt angles and the solar irradiance on PV module in Tanjung Morawa, North Sumatera, Indonesia. A mathematical modeling is used to calculate the tilt angles and the solar irradiance, which depend on latitude and day number. The optimum tilt angle of PV module is determined by searching a yearly maximum total and average solar irradiance. The tilt angles of PV module in Medan, are -19.920 to 26.080. The positive, zero and negative tilt angles indicate that the PV module is inclined to face south, on horizontal surface and north, respectively. The yearly minimum, maximum and average solar irradiance are 988.5 W/m2 and 1123 W/m2 and 1061 W/m2, respectively. For Tanjung Morawa, North Sumatera, Indonesia (has latitude 3.53 0 N) is recommended that the optimum tilt angle of the PV module from the horizontal be put equal to 5.080 (an angle of the latitude of the location +1.550).