Demand Response is used to reduce the mismatch between electricity generation by renewable energy sources and electricity demand. Buildings are large energy consumers and can play an important role in matching generation and demand. Energy flexibility is defined as the ability of a building to manage its generation and demand according to local climate conditions, user needs and grid requirements. This article focuses on the thermal mass of buildings as a source of energy flexibility. The investigation is based on a characterization assessment method on single building level and on aggregator level. On single building level, different building parameters such as the thermal capacity of the construction, window to wall ratio, façade insulation and glazing properties are investigated and their impact on the energy flexibility is assessed. The thermal capacity and window to wall ratio are considered the most influential in the studied buildings. The characterization assessment shows a minimum 0.14 and a maximum 0.34 kWh/m2 potential shiftable energy. The results of the characterization assessment method are compared with Demand Response events under more realistic conditions for specific case study buildings. The results show to be representative of the potential shiftable energy of a building envelope. The results show that in certain cases, the potential shiftable energy by the thermal mass of the building is comparable to the capacity of a commercial battery for utility buildings (232 kWh). Finally, the aggregated flexibility of clusters of Dutch office buildings is investigated and analogized to different electrical battery storage projects around the world. The results show that the thermal mass of 17 reference buildings show the potential of a storage capacity close to the biggest electrical battery projects in the Netherlands. The limitations of this study and advantages and disadvantages of electrical storage and thermal mass storage are discussed.