Thermal energy storage (TES) tanks for cold storage can be used for peak load shaving. This paper presents and evaluates a mathematical model where a TES tank is filled with commercial phase change material (PCM) flat slabs. The 2D model is used for simulating the outlet temperature of the heat transfer fluid, and also the heat transfer rate during the discharging process. The study includes the use of an approximation of the PCM specific heat (cPCM;old) which corresponds to the parameter calculated in the previous iteration of the implicit finite difference method. Thus, an evaluation of the different model parameters is performed, based on the comparison between the computational time and accuracy of the different simulations. Moreover, the uncertainties in different input variables are also analysed in order to find out which variable should be known more accurately. The results show that using the approximated parameter cPCM;old is a good solution for reducing the computational time despite a slight error increase in the case of using small time step in the simulation. Moreover, the inlet HTF temperature, and melting temperature, density and specific heat of the PCM are the main parameters to take into account in terms of uncertainty variables evaluation.