In books and various websites, I have seen that the reactive power is measured using two wattmeter method. This method is applicable to a balanced three-phase system.

But at home, just a single phase is supplied. And we use different types of load. For example water pumps, electric fans, LED, etc. Is there any way to find the value of the imaginary power? The load impedance is also unknown. (As it varies time to time)

  • 1
    \$\begingroup\$ are you done with this question and answer now? \$\endgroup\$
    – Andy aka
    Nov 30, 2020 at 11:44
  • \$\begingroup\$ This question is done too. \$\endgroup\$
    – Sadat Rafi
    Nov 30, 2020 at 11:47
  • \$\begingroup\$ The concept of reactive power only makes sense in steady-state. If you have changing impedances (what you stated in your question), then you have transient state, and the formula for reactive power is not applicable. Furthermore, the answer by Andy Aka is valid in sinusoidal steady-state (without harmonics), not in non-sinusoidal steady-state (with harmonics); LEDs and other non-LTI (linear time-invariant) devices introduce harmonics. \$\endgroup\$
    – alejnavab
    Jun 2, 2021 at 7:29
  • \$\begingroup\$ Yeah. I understood that it will work for pure sin wave. I saw in an application note of NXP that the voltage, current and phase is measured from the DFT sequence. \$\endgroup\$
    – Sadat Rafi
    Jun 3, 2021 at 3:49

1 Answer 1


Measure the real power and also calculate the apparent power from V and I (RMS values) then, use Pythagoras to compute the other side of the triangle (the so-called reactive power): -

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