A sewage treatment plant simulator was used to evaluate the effects of energy saving and energy production measures on reducing power consumption and GHG emissions at four medium-sized sewage treatment plants with treatment capacities ranging from 50,000 m³/day to 100,000 m³/day. When introducing energy-saving measures aimed at reducing the amount of aeration air, the energy-saving effect was 4.5-18.6%, less than 20%. On the other hand, with energy production measures, the self-sufficiency rate was expected to increase from 11 to 50% by introducing digestion gas power generation, and it was calculated that the self-sufficiency rate would improve to 21 to 88% by combining excess sludge solubilization and MLSS reduction measures. Simultaneous implementation of energy saving and energy production measures further improved the self-sufficiency rate to 24-99%, and approximately the same effect was expected over the year. When evaluating the GHG reduction effect using the CO2 offset rate, the CO2 offset rate was 57% even at A-STP, which had almost reached power independence, with direct emissions remaining from water treatment. The CO2 offset rate of B-STP for nitrification suppression was 26%. C-STP, with sludge incineration, had the second highest CO2 offset rate at 43%, as the amount of sludge to be treated was reduced due to the introduction of the anaerobic digestion for energy generation. The CO2 offset rate of advanced D-STP was 19% because of the advanced treatment.