Advanced sizing of the thermal wing ice protection system (WIPS) requires an improved and a robust manner to simulate the system operation in unsteady phases and particularly in de-icing operations.A two dimensional numerical tool has been developed to enable the simulation of unsteady anti-icing and de-icing operations. For example, the WIPS may be activated with delay after entering into the icing conditions. In this case, ice starts to accrete on the leading edge before the WIPS heats up the skin. Another example is the ground activation of the WIPS for several seconds to check its functionality: low external cooling may cause high thermal constraints that must be estimated with accuracy to avoid adverse effects on the structure.Thermal de-icing WIPS integrated in composite structures intrinsically have unsteady behaviors; the tool enables the computation of the skin temperature evolution with the time. This provides an insightful indication on the de-icing performance of the WIPS and allows adjusting the design of the WIPS to optimize the system energy consumption. Calibrations through IWT were conducted to ensure the accuracy of the global thermal prediction.Regarding structural sizing, the modelling provides temperature distribution mapping of the structure. This is advantageous for assessing the impact of thermal ageing of metallic structures or for validating the used materials of composite structures. Therefore the weight can be optimized.