An increased wind capacity penetration reduces loading on conventional thermal units causing higher fuel requirements due to the off-design operation. Regarding the environmental analysis, such an adverse effect should be allocated to the operational phase of wind turbines. In the present work, we apply Thermo-ecological Cost (TEC) to evaluate the environmental performance of wind power systems operating in Poland and Italy. The analysis focuses on the quantitative assessment of the effect of additional chemical energy consumption due to part-load operation of the conventional power units in both analyzed electricity systems. We present the results for two different dispatch strategies. The results confirm high environmental effectiveness of wind power systems. However, the TEC resulting from the compensation for wind generation variations has a significant contribution to the overall LC-TEC index. In particular, without considering the effect of compensation, the TEC for wind turbines are from 47 to 65 times lower than for coal-fired power plants and 35 to 48 times lower than for NGCC plants. Concerning the real load conditions, and considering the effects resulting from the compensation for wind generation variations, the TEC index for this phase contributes between 36% and 75% to the total TEC value. (C) 2018 Elsevier Ltd. All rights reserved.