The "Solar Complex of Plan-les-Ouates" is a traditional multifamily building with some commercial and administrative areas. It was designed to consume a minimum amount of thermal energy by combining several renewable energy systems (1400 m(2) of solar roof, buried pipe and exhausted air heat exchangers) with an optimised envelope and electrical equipment. Initially predicted to consume 160 MJ/m(2) per year of gas, a gas energy use index (per unit heated floor area) of 246 MJ/m(2) per year was measured. The energy analysis of the building, based on a 3-year period of monitoring, brought up the most relevant points that explain this difference: the real conditions of utilisation (such as the interior temperature) and the real performance of the complete technical system are not taken into account in the theoretical value. Both technical and economical aspects of the renewable energy systems were analysed in detail. An important lesson learned from this experiment is that the energy concept of buildings must be simple and consistent and the complexity of the technical installations must be carefully managed from the design-stage to the exploitation. Detailed monitoring of innovative low-energy buildings is recommended to understand the possible discrepancies between theoretical and real heat consumption and to improve the transfer of new energy technologies to large-scale real constructions. (C) 2004 Elsevier B.V. All rights reserved.