We present a digital sampling algorithm that uses two high resolution integrating voltmeters that are synchronized by a phase-lock loop clock to accurately measure the active and apparent power parameters for sinusoidal power measurements. The developed system provides high precision measurements of the root mean square, phase and the complex voltage ratio of the ac signal with sophisticated algorithm which process the data in frequency domain. The system was designed to be used at the Korea Research Institute of Standards and Science as reference power standard for electrical power calibrations. The detail uncertainty calculations have shown that the accuracy of the measurements were better than 4 μW/VA (for k = 1) and the level of uncertainty was valid for the power factor range 0 ~ 1 for both lead and lag conditions. The system is fully automated and allows power measurements and calibration of high precision wattmeters and power calibrators at the main power frequencies of 50 and 60 Hz.