The objective of this work is to investigate the daily and the instantaneous performance of a linear Fresnel reflector (LFR) experimentally and numerically. The examined LFR has an inverted flat plate receiver with a 36 m(2) total collecting aperture area and is coupled to a storage tank of 1 m(3) volume. This system is investigated experimentally for daily operation in the period from May to October for the climate conditions of Athens (Greece), using water as the working fluid (temperature levels up to 100 degrees C). Moreover, a detailed mathematical model for the prediction of the instantaneous and of the daily performance is developed. This model is validated with the experimental results for 52 operating days and it estimates the daily useful energy production with a mean error of 3%. This modeling is based on the energy balance on the collector loop and the storage tank. The emphasis is given on the determination of the optical losses of the collector, which are associated with the sun position during the day. It is found that the maximum useful heat production is about 8.4 kW, while the maximum daily heat production is about 260 MJ. The results of this work can be exploited for the evaluation of integrated solar concentrating systems and especially of linear Fresnel reflectors.