Buildings can facilitate a safe and uninterrupted transition from traditional production-respond to future demand-respond energy systems by delivering flexibility service to the system with smart control of their energy loads. Current methods used for building design and performance evaluation are focused on minimizing the building energy use and do not quantify the flexibility potential of the buildings. As the energy flexibility is of interest for the built and network environment, this paper presents a simple energy flexibility factor, which combines the needs of both and can be easily applied during the simulation/design and operation phases of a building.