This paper reports thermal performance of porous latent heat thermal energy storage (or LHTES) system filled with nano-phase change material (or nano-PCM). In the first part of the paper, scale analysis is executed to estimate the extent of the complete phase change process which is a key factor of designing LHTES systems. The scale analysis results in simplified relationships among different non-dimensional parameters (i.e., Fourier number, Stefan number, Rayleigh number, Nusselt number, porosity of the porous medium, and nano particle volume fraction). In the second part, the natural convection melting process of nano-PCM inside the porous medium is solved numerically. For the porous medium the Darcy model is employed. The numerical simulation serves two purposes: (i) it verifies the correctness of the relationships proposed by scale analysis in the first part of the paper and (ii) it identifies the effects of nano-particle volume fraction, time, and Rayleigh number, on flow field, thermal field, and heat transfer process during the melting of nano-PCM inside the thermal energy storage system. The proposed scaling relationships can be applied to predict the progress and execution of LHTES system filled with porous medium saturated by nano-PCM. (C) 2015 Elsevier Ltd. All rights reserved.