A formula of synthetic errors is obtained from analysis of effect of synchronous error and quantification noise on the measurement precision of the electrical parameters in extracting an effective value and active power by using a weighted algorithm. The results demonstrate that the measurement errors caused by asynchronous sampling decrease rapidly even to a secondary factor, and the quantification errors may become a key factor of errors with the increase of the order number of the weighted function used. The measurement errors of the electrical parameters caused by quantification noise of data are in direct proportion to the precision of quantification and in inverse proportion to the square root of the number of the sampling points. The measurement errors caused by data quantification can be decreased by increasing the number of quantification bits of the data or the number of sampling points.