Calorimetric measurement systems are commonly used to determine the thermal transmittance or U-value of fenestration specimens, particularly for complex systems with additional external shadings, embedded photovoltaic cells or non-homogeneous patterns. In the past, measurements were mainly performed under winter conditions, where heating of buildings is required, but for tropical climates or summer conditions, where cooling of the building is required, very few thermal transmittance data are available, as most of the calorimetric systems were optimized for the winter conditions only. This paper presents a calorimetric hot box (CHB) for summer conditions complying with international standards, but with advanced measurement methodology and uncertainty analysis model. It includes the measurement results of double glazing units and comparison with simulation results obtained with WINDOW and THERM software. The comparison revealed a difference of less than 5% which can be considered negligible as it falls within the accepted uncertainty. Further results from measurements of complex fenestration systems with semi-transparent thin-film photovoltaics embedded into laminated and double glazing units are also presented. Hence, the presented system and method can pave the way for thermal performance validation of standard and complex fenestration systems necessary for energy efficient buildings in the tropics. (C) 2012 Elsevier B.V. All rights reserved.