In this paper, the effects of ambient temperature and relative humidity on performance of wet, dry, and two combined cooling systems are studied for a typical 250 MW power plant in Hamedan, Iran. Although there have been many works in background of effects of different parameters on performance of cooling towers, there is no detail analysis about different alternatives of hybrid cooling systems due to the water consumption. In this study, four alternatives were considered for the cooling system: wet, dry, and two combined wet and dry cooling systems. One of the hybrid systems demanding only half of the cells of the existing wet cooling system so has a limitation for water consumption. Another one has no limitation for utilizing all of the cells of the existing wet cooling towers and as a result does not have any limitation for water consumption. Investigating mentioned cases are significant due to the lack of water in middle-east countries. Also, by means of monthly profiles of ambient temperature, the amount of annual power loss is computed for each case. The water consumption for each case, is computed as well. Finally, the best alternative was determined by computing both the capital and annual costs, and annual water consumption.