In wastewater treatment plants (WWTPs), the majority of energy inputs is consumed by aeration systems to support both the biochemical oxidation of organics and transformation of ammonia-nitrogen into nitrate-nitrogen. Consequently, WWTPs energy efficiency evaluation only based on metrics derived from the organic constituents such as chemical oxygen demand (COD) or biological oxygen demand (BOD) may not reflect the true energy consumption of WWTPs with variable influent quality. Therefore, to overcome this limitation, total oxygen demand (TOD) is introduced in this article, and a novel index E-o, namely the energy consumption for the removal of a unit mass of TOD is proposed for evaluating the energy efficiency in WWTPs. Furthermore, by considering the stoichiometric relations of oxygen consumption for the oxidation of both organics and ammonia nitrogen, methods for calculating the E-o are proposed. Using the novel E-o index and the available annual operation data of 2022 WWTPs, the current status of energy consumption for wastewater treatment in China were analyzed. The findings show an average E-o decrease from 5.2 kWh/kg to 1.2 kWh/kg as the WWTP loading rates increase from 20% to 100%. Also, E-o decreased from 4.1 kWh/kg to 1.5 kWh/kg as the average TOD removal increased from 60% to over 90%. Moreover, E-o decreased from 2.9 kWh/kg to 1.0 kWh/kg as the WWTP scale increased from less than 10,000 m(3)/d to over 5,00,000 m(3)/d. Thus, the energy efficiency of WWTPs increases with increasing loading rates, TOD removal, and scale. Also, the wastewater treatment technology applied influences the E-o significantly, especially for small-and medium-size WWTPs with capacities less than 50,000 m(3)/d which account for circa 76% of all WWTPs in China. The WWTPs applying sequential batch tractors (SBR) tended to show lower average E-o (< 1.7 kWh/kg) than those applying anaerobic/oxic (A/O), anaerobic/ anoxic/oxic (A(2)/O) and oxidation ditch (DO) (1.9 kWh/kg <= 5 E-o <= 2.0 kWh/kg). Thus, as an index of the energy consumption per unit mass of TOD removed, E-o reflects the essence of wastewater treatment for pollutants removal in contrast to other existing energy indices based on the volume of treated wastewater. Moreover, due to the large variability of the WWTPs influent qualities, the TOD contributed by ammonia-nitrogen varied widely between 12.2% and 80.7% of the total TOD. Therefore, E-o calculation based on TOD but not merely the organic component (COD or BOD) provides a more comprehensive index for evaluating and optimizing the energy efficiency of WWTPs.