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A review of the two-wattmeter method for measuring real power is given, emphasising its advantages in responding immediately to changes in real power flows and eliminating the double-frequency term. The usual technique for determining reactive power is described, showing the presence of a DC term plus the troublesome double-frequency term. Theory is developed for determining reactive power flow that eliminates the double-frequency term and produces an instantly responding DC term only. Five measurement techniques are shown to be possible, three of which are of practical value. Whereas two of the five methods require a neutral connection for voltage measurement, the methods selected for practical realisation require line voltages only. The applicability of the chosen techniques to include the two-wattmeter method of real power measurement is described. An optimal choice of the techniques is suggested, based on minimal use of electronic components and longevity of calibration. A series of experimental results comparing the various techniques of fast reactive power measurement is presented, confirming the theory and showing some practical limitations to elimination of the double-frequency term.