Abstract. The characteristics of absorber plate of a flat plate solar collector play an important role in the improvement of the performance. In this work, a numerical analysis is carried out to explore the effect of absorptivity and emissivity of absorber plate to the performance of the solar collector of a solar water heater. For a results comparison, a simple a simple solar box cooker with absorber area of 0.835 m  0.835 m is designed and fabricated. It is employed to heat water in a container by exposing to the solar radiation in Medan city of Indonesia. The transient governing equations are developed. The governing equations are discretized and solved using the forward time step marching technique. The results reveal that the experimental and numerical results show good agreement. The absorptivity of the plate absorber and emissivity of the glass cover strongly affect the performance of the solar collector. IntroductionIn order to avoid the catastrophe of the globe due to global warming, emission of Greenhouse gas gases to the atmosphere must be reduced. One of the potential solutions is to enhance the use of renewable energy resources. Solar energy is known as a potential renewable energy resource. It is convinced, it will play an important role in the future energy demand. In a year, the globe is exposed by 3,400,000 Exa-Joule of solar energy. This is a vast amount of energy. It can fill the present world energy consumption by only 1 hour 15 minutes of irradiation [1]. However, at the present time, only a very little portion of the human energy consumption is filled with the solar energy. According to REN21, since 2030 solar energy application will increase significantly. Thus, studies on solar energy have increased significantly [2]. Industry related to solar energy application is developing constantly in all over the world. This is because of twofold, increasing energy demand and limitation on major energy source the fossil fuel. Indonesian islands have a big potency of solar energy. It is also constant in every year [3]. The solar energy can be harvested by solar photovoltaic and solar thermal methods. In the solar thermal method, the solar energy can be collected and used as a heat energy for several applications such as solar drying process [4], solar adsorption refrigeration [5], solar desalination [6,7,8], solar water heater [9], solar cooker [10], etc.