The rationalization of energy at the district scale requires the development of numerical tools capable of simulating not only the energy demand, but also energy loads at such a scale. However, such simulation remains difficult because of the computational cost and lack of reliable data. A common solution to both these problems is to simplify the model, generally at the expense of accuracy. To overcome these problems, a specific modeling and simulation platform called Modular District Energy Model (MoDEM) is developed. This platform enables the calculation of the heating and cooling load curves of each building, integrating the interactions between the buildings taking into account longwave radiative heat transfers between buildings external surfaces. Moreover, with different levels of models included in its calculation engine, MoDEM also enables the testing of modeling assumptions. MoDEM can also handle the geographical information system geometry and detailed solar radiation calculation. To validate the application of MoDEM, a district in Paris is modeled, comprising 900 flats built in the 1930s. The results demonstrate that 1) various models have little impact on energy consumption and 2) some physical phenomena usually not modeled appropriately, such as the longwave radiative exchange between buildings, can no longer be neglected while considering the load curve. It is worth mentioning that MoDEM is based on a co-simulation layer allowing parallel computing. From a broader point of view, this specific feature enables the easy and computationally efficient integration of various models like, for instance, specific electricity consumption, renewable energy production, grid management simulation and energy storage.