A device (100; 200, 400) for measuring power of an alternating current with respect to a selected frequency, comprising: a frequency detector (102, 402) having a first oscillator to generate a local reference signal to detect a fundamental frequency of the AC; and a power analyzer (208, 404, 408) to analyze power of the AC, the power analyzer comprising: a second oscillator (120, 210) to receive from the frequency detector a signal indicating the detected fundamental frequency of the AC and generate first and second substantially mutually orthogonal sinusoid signals at the selected frequency using the signal indicating the detected fundamental frequency of the AC; a first group of multipliers (122, 124, 126, 128) for mixing the first and second sinusoid signals with a current data signal and a voltage data signal of the AC respectively; a group of low-pass filters (130, 132, 134, 136) for respectively removing high frequency components from a multiplication product of the first group of multipliers; a second group of multipliers (138, 140, 142, 144, 146, 148, 150, 160) for respectively mixing the filtered multiplication products; and a plurality of adders (152, 154, 156, 158) each to sum together a pair of multiplication products of the second group of multipliers. a frequency detector (102, 402) having a first oscillator to generate a local reference signal to detect a fundamental frequency of the AC; and a power analyzer (208, 404, 408) to analyze power of the AC, the power analyzer comprising: a second oscillator (120, 210) to receive from the frequency detector a signal indicating the detected fundamental frequency of the AC and generate first and second substantially mutually orthogonal sinusoid signals at the selected frequency using the signal indicating the detected fundamental frequency of the AC; a first group of multipliers (122, 124, 126, 128) for mixing the first and second sinusoid signals with a current data signal and a voltage data signal of the AC respectively; a group of low-pass filters (130, 132, 134, 136) for respectively removing high frequency components from a multiplication product of the first group of multipliers; a second group of multipliers (138, 140, 142, 144, 146, 148, 150, 160) for respectively mixing the filtered multiplication products; and a plurality of adders (152, 154, 156, 158) each to sum together a pair of multiplication products of the second group of multipliers. a second oscillator (120, 210) to receive from the frequency detector a signal indicating the detected fundamental frequency of the AC and generate first and second substantially mutually orthogonal sinusoid signals at the selected frequency using the signal indicating the detected fundamental frequency of the AC; a first group of multipliers (122, 124, 126, 128) for mixing the first and second sinusoid signals with a current data signal and a voltage data signal of the AC respectively; a group of low-pass filters (130, 132, 134, 136) for respectively removing high frequency components from a multiplication product of the first group of multipliers; a second group of multipliers (138, 140, 142, 144, 146, 148, 150, 160) for respectively mixing the filtered multiplication products; and a plurality of adders (152, 154, 156, 158) each to sum together a pair of multiplication products of the second group of multipliers.