Because of the introduction of renewable energy sources, the energy system has become more future proof but more pressure has come on the reliability and the affordability. The major problem of renewable energy is that the supply does not match its demand. The missing link towards an energy neutral built environment is energy storage. In this thesis, a model is proposed for one specific thermal energy storage system called the ‘Ecovat energy storage system’. This system is the first product that is able to store high temperature (up to 90°C) efficiently over a longer period (>6 months) with a loss of heat in this period less than 10%. The purpose of the model is to indicate the effect of different renovation scenarios for a given apartment building on the Ecovat energy storage system. Four renovation scenarios are developed. For each scenario the following variables are optimised: the size of the storage vessel, the power capacity for each installed device, and the number of PVT panels. The results indicate the impact of the different renovation scenario and the different Ecovat vessel sizes on the total Ecovat energy storage system. Based on the total cost of ownership the most optimal renovation scenario is determined.