This paper presents a method for estimating the total heat loss coefficient, the total heat capacity and the gain factor based on measured data for the internal-external temperature difference, the domestic load and the supplied heat. Knowledge of these performance parameters is essential for a reliable energy demand forecast, close guidance and the accurate analysis of efficiency actions in buildings. The method was validated on measurements from a test cell. The values obtained for the performance parameters were in good agreement with a lumped capacitance analysis of the heating and cooling of the test cell. The deviation in the total heat loss coefficient, expressed in terms of the root mean square error, was between 2.5 and 9.4%. The values obtained for the total heat capacity were on average 9.8% higher than the reference value and for the gain factor the average deviation was 12.5%. The method shows promising signs of becoming a robust and accurate tool for extracting both the magnitude and the variation of the performance parameters, based on easily accessible data. (C) 2004 Elsevier B.V. All rights reserved.