Public swimming pools have an important energy consumption and are rarely optimized. One way to do so is to develop a model able to represent the swimming pool behaviour (water temperature and energy consumption) whatever the climatic conditions. The evaluation of thermal losses is done in this work thanks to the presented model and to measurements (ambient air and pool's temperature, relative humidity, direction and velocity of wind, global solar irradiation) performed on a public outdoor swimming pool situated in La Reunion, a French overseas department located in the Indian Ocean close to Mauritius). Each contribution to the decrease of water's temperature is clearly identified and represented, leading to quite accurate comparisons between simulated results and tests for 4 different nights.