Every existing source of energy is affected by climate variability. The most direct climate impacts are related to power plant cooling and water availability. Local weather conditions affect the capacity of cooling towers and natural water bodies to transfer waste heat from steam condensers to the atmosphere. Under hot summer conditions even the inlet water from a river may approach the limit set for discharge, and this will mean that the plant is unable to run at full power. For the given thermal power plant configuration, cooling water temperature change generates changes in the condenser pressure. Those changes have a great influence on the power output of the plant. This effect, due to daily variations of the air humidity and temperature, is even more marked for the close cycle cooling (with cooling towers). Without technology-based improvements in cooling system efficiency, the steam-cycle energy efficiency would decrease. In this article, the overview of cooling water temperature increase impact on the energy efficiency in Serbian power plants is given, based on meteorological data and numerical simulation. Obtained results could be used as useful guidelines in design of the new power plants and also in improving existing power plants' performances.