With the popularization of new types of electrical equipment, the load of the power system has become increasingly complex, and the nonlinear characteristics have become more prominent. However, the traditional definition of three-phase apparent power is not unified and cannot truly reflect the energy utilization rate, which cannot meet the requirements of most power consumption conditions. Based on the core essence of apparent power, this paper starts from the equivalent process and engineering approximation of apparent power in a single-phase power system, elaborates on the equivalent principle, calculation method and practical significance of effective apparent power, and extends it to a three-phase power system. At the same time, a three-phase intelligent apparent power meter supporting arithmetic, vector and equivalent algorithms has been developed based on the effective apparent power theory. Its measurement accuracy and engineering applicability have been verified through a verification device, a traceability circuit and an LED nonlinear load test. The results show that the measurement uncertainty of the apparent energy of this electricity meter does not exceed 0.08%, which can provide a reliable basis for energy efficiency assessment and power quality management in smart grids.