Energy consumption is steadily rising globally, especially in buildings. When moving towards a greener approach in constructing new buildings, it is essential to target the existing building stock through retrofitting. Dynamic Building Energy Simulation (DBES) modelling helps designers and professionals estimate the potential amount of energy saving in buildings. Although DBES plays a major role in quantifying energy demand, a performance gap, defined as the discrepancy between calculated and actual energy savings, remains a concern for professionals. To overcome this performance gap, calibrating DBES models is a prerequisite before using the model to estimate energy savings through specific measures. The commonly used calibration method is a statistical one that depends on indices related to energy bills. As the energy crisis worsens, users are shifting towards cheaper energy sources and using a combination of energy sources to meet their needs. Hence, these statistical indices that depend on energy bills are not easily applied in many cases, especially in countries where a combination of fuels is sometimes used for heating, cooking, and heating water. In this paper, the heating load in an apartment in Hebron, Palestine, was modelled as an internal gain. Subsequently, the model was calibrated using internal environment measurements (internal temperature) taken during two monitoring phases for an apartment in Hebron, Palestine. A validation method for an apartment was presented using internal temperature data from the monitoring phases. The results demonstrate that the model was calibrated and is a reliable tool for optimisation.