This study presents an experimental investigation of thermodynamic performance of the sensible thermal energy storage in water filled PET bottles. Heating of the indoor sports hall with solar energy have been researched experimentally. 5120 pieces 1.5-1 water filled PET bottles are used in thermal energy storage unit. Energy charging period is between 7:00 and 17:00, while the energy discharging period is between 17:00 and 22:00. Performance of the sensible thermal energy storage in water filled PET bottles has been examined thermodynamically for October and November. Overall energy and exergy efficiencies have been calculated as 79.85% and 51.89% for October and 69,95% and 46.26% for November, respectively. Consequently, it has been found that using water filled PET bottles as storage medium in solar energy heating systems is a convenient method. Because, the use of water filled PET bottles do not require an extra heat exchanger between heat transfer fluid and storage medium.