The establishment of an optimal mix of renewable energy sources (RES) is pertinent to creating low-carbon energy systems with adequate load-following capabilities. Transitioning from a fossil fuel-based energy system to a system based on RES is a process in which the RES penetration must be guided by specific targets in the medium term. Moreover, the relative contributions from RES and fossil fuels in every stage of the transition process must be identified through clear optimization criteria. In this article, we propose a methodology denoted the Minimum Total Mix Capacity (MTMC) to determine the optimal mix of RES and fossil fuels in an electricity system by taking into account the hourly values of RES production and electricity demand. The MTMC methodology is applied to the Mexican electricity system, which greatly depends on fossil fuels. The Mexican Congress mandated that the fossil fuel-based electricity generation be limited to 65% by the year 2024, 60% by 2035 and 50% by 2050. We use the MTMC methodology to investigate the potential for RES integration into the Mexican electricity system by constructing three scenarios (low, mid and high biomass) to achieve the 2024 target and analyze the system's response to varying contributions of wind and solar power production to the scenarios. The minimum complementary capacity based on fossil fuels that is needed to cover the demand without electricity imports is assessed to determine the total mix capacity for the transition system. Using the MTMC methodology, several combinations of biomass, wind and solar power that achieve a minimum of 35% RES electricity production are identified; however, there is only one combination that results in the minimum overall capacity, hence resulting in the optimal mix. Moreover, a domain of near-optimal mix combinations can be identified among all of the scenarios, which may also be considered as an alternative. Biomass has the highest effectiveness in terms of high RES production and the lowest required overall power generating capacity. (C) 2015 Elsevier Ltd. All rights reserved.