In the operational diagnosis of air-source heat pumps, the conventional coefficient of performance has limitations in distinguishing environmental effects from equipment performance degradation because of its dependency on the operating conditions. To address this issue, this study proposes a performance consistency index based on the second law of thermodynamics. By normalizing the second law efficiency under actual operating conditions against its value under rated conditions, a diagnostic method is established for energy efficiency based on this index, with operational robustness and design benchmarking capabilities. The study demonstrates that the performance consistency index inherits the stability and comparability of second-law efficiency, effectively decoupling the effects of environmental parameter fluctuations and equipment performance degradation. It is independent of the operating conditions and accurately characterizes the equipment status. With a design benchmark based on performance consistency index PCI = 1, the deviations between the actual performance and design objectives were quantified. Validation through operational diagnosis of four units confirmed that the performance consistency index successfully identified the degradations in partial-load energy efficiency and low-temperature performance. The performance consistency index provides a theoretical tool for the long-term energy efficiency monitoring of air-source heat pumps, enabling the precise identification of inefficient units through threshold settings and supporting scientific decision-making for equipment renewal in "coal-to-electricity" projects.