An apparatus and method thereof are described to measure alternating current (AC) power, including: an AC voltage measurement circuit, an in-phase/quadrature (I/Q) direct down converter, an analog/digital converter, and an AC power calculator. The AC voltage measurement circuit is configured to measure first and second AC positive voltages and first and second AC negative voltages from an AC voltage source. The I/Q direct down converter is configured to directly convert each of the first and second AC positive voltages and the first and second AC negative voltages into an I-signal and a Q-signal orthogonal to each other using down conversion. The analog/digital converter is configured to convert the I-signal and the Q-signal into I-data and Q-data, respectively. The AC power calculator is configured to calculate the AC power using the I-data and the Q-data.