The utilization factor is a relative well-known concept used in some simplified calculation methodssuch as the procedure described in the CEN EN-13790 where the FU depends on the inertia of thezone. In this paper we will analyse the inertia influence of each component of the zone in the inertia ofthe zone itself. With the results of this study, we will be able to know where to act if we desire to ariseor diminish the whole thermal inertia of a room. The present paper starts analysing the energy storedand released in building components as walls. In this step we will establish a charge period and adischarge period. In the load period the component will storage heat while in the discharge period thecomponent will release heat. The duration of each couple of charge/discharge periods is 24 hours. Ifwe repeat this period a sufficient number of times we will obtain a cuasi-steady-state. In this way wewill be able to assess the restored energy in the component.The energy restored has been studied in three scenarios that successively lead to the actual, morerealistic, situation. Establishing a comparison between the results for the different scenarios we couldknow the influence of the conductivity and the influence of the energy losses. Up to this point we havequantified how much heat is restored to the indoor space adjacent to the component as aconsequence of the existence of solar radiation. Further from here we will explain how the heatrestored is used to diminish the heating requirements revising the concept of utilization factor. In thisstep we will analyse all the concerning parameter making special attention to the inertia of thecomponent. Finally we will determine the influence of the inertia of each component in the total inertiaof the room.