Solar thermal power plants are a promising way of providing renewable electricity. Unlike other renewable energy technologies, solar thermal power plants have the ability to store thermal energy, which decouples the electric power generation from hours of available solar irradiation. Today, the only applied commercial thermal storage system is a two-tank sensible heat thermal storage based on molten salts. This work focuses on the transient heat loss modeling of such molten salt thermal energy storage tanks. More precisely, this paper proposes a degree of physical modeling which describes the governing heat transfer mechanisms sufficiently accurate, obtaining a suitable tool for concentrating solar power (CSP) performance simulations, applicable for parabolic trough collector, as well as for central receiver plants. A fully transient storage tank model has been developed and simulated over 3 sets of 6 reference days, providing typical weather conditions of a solar thermal power plant location as model input. The model has been successfully compared to heat loss data that was observed at real application molten salt storage tanks. Several modeling assumptions, used heat transfer correlations and simplifications are outlined. Modelica has been used as modeling language. (C) 2013 Elsevier Ltd. All rights reserved.