State estimation is a crucial and basic function in energy management systems. The true state of a real power system is unavailable and it is therefore of cardinal importance to assess the reliability of the results from state estimators. Drawbacks of the widely used acceptance rate index are elaborated

and a new index to assess the reliability of state estimation results is proposed

which is based on the consistency rate and mismatches in zero injections. A simple 9-node system is employed to illustrate drawbacks of acceptance rate and advantages of the proposed index. Extensive simulations on a 118-node system have statistically demonstrated the effectiveness of the proposed index. Finally

an instance from actual power systems is used to show the problems of acceptance rate and the practical value of this research.