The light is one of the important factors for the microalgae growth in the biofuel technology. As biofuel project is large and expensive thus before setting a microalgae based biofuel project in any geographical location, it is important to investigate the suitability of all important parameters involving with the system. This paper aims to investigate the sunlight availability and the microalgae growth for a photobioreactor at Chittagong University of Engineering and Technology (CUET). A computational growth model related to the average irradiance is proposed to calculate the growth of microalgae. We observed that average irradiance is the highest in June and is the lowest in December. From our simulation it is found that the growth of microalgae varies with the average irradiance in a year.