With the rapid growth of the electricity produced by RES (renewable energy sources), especially those highly variable and unprogrammable (e.g. wind and solar power), the need of energy system flexibility increases significantly. Since RES currently represent a significant fraction of the power supply, their variable nature poses challenges to power grid operation, such as RES curtail and loss in global efficiency of thermoelectric plants, since they are often operated at part-load as fluctuating back-up power. In particular, thermoelectric plants recently moved their role from base-load power to fluctuating back-up power. Such a cycling operation represents a less obvious effect of grid flexibility requirement due to RES penetration. Main effect is the increment of both energetic costs, due to reduced efficiency operation, and wear-and-tear costs. This aspect is deeply analysed in reference to the Italian electricity generation mix in the period 2008 -2012. Moreover, the possible coupling of energy storage systems with thermoelectric plants is highlighted as an alternative solution respect to retrofitting of existing plants. (C) 2015 Elsevier Ltd. All rights reserved.