To tackle the climate crisis, the European energy strategy relies on consumers taking ownership of the energy transition, accelerating decarbonisation through investments in low-carbon technologies and ensuring system stability and reliability by actively participating in the market. Therefore, tools are needed to better understand an increasingly complex and actor-dense energy system, tracking socio-technical dynamics that occur at its margins and then predicting the effects on larger scales. Yet, existing domestic energy demand models are not flexible enough to incorporate a wide range of socio-technical factors, and to be incorporated into larger energy system simulation environments. Here, a co-simulation design for domestic energy demand modeling is presented and motivated on the basis of four design principles: granularity, scalability, modularity and transparency. Microsimulation of domestic energy demand, through the Python open source library demod, shows that it is possible to achieve high detail and high temporal resolution without compromising scalability. Furthermore, mosaik, an open source co-simulation framework, makes it possible to generate, integrate and orchestrate a multitude of demod-based instances with other independent models, which for the illustrative purposes of this study are represented by a heat pump model. The authors hope that the detailed documentation of the proposed solution will encourage interdisciplinary and collaborative efforts to develop a simulation ecosystem capable of investigating alternative energy transition pathways and evaluating policy interventions through the socio-technical lens.