Category Archives: Power Factor

Dr InPhase Edu Series – An Exercise in Reactive Power Calculation

Doctor_Character-04-1Good day every body, Today on our Edu Series we will focus on some practical knowledge related to Reactive Power calculation.  Calculating your reactive power is critical as it can help make sure that your design is reliable and you get a reliable power factor output.

Consider an electrical network with the details below,

Demand = 2500kVA; Initial PF1 = 0.76; Desired PF2=0.97.

The reactive power required to achieve the desired PF (0.97), is calculated as below:

We know that

Where φ is the angle between voltage and current vectors.

From the above,

Now here arises the doubt. We say, the above method of calculation is flawed. There is a fundamental electrical engineering mistake. Before you go on any further, we would like you to think and comment in the comments section below if you find the flaw. Because once you scroll down, you are likely to lose your unique perspective on the above. Therefore, please comment and proceed for the answer below. This can help knowledge sharing with numerous angles / perceptions. Thanks in advance for that.


The mistake is in Point ②. We have considered the same kVA for ① where the pf is 0.76, and② where the pf is 0.97. When actually, it is Real Power or kW which remains constant. Load remains unchanged, which in other words translates into constant kW and NOT constant kVA.  The same is shown in the diagram below:

From the above diagram, we know now the ‘kvar’ required to improve the system from PF1 to PF2 (i.e, 0.76 to 0.97) is,

Applying Pythagoras theorem for the above diagram,


Which implies, [kvar1 = kW × tan φ1] and [kvar2 = kW × tan φ2]. Substituting these in equation ④,

In terms of power factors,

This is a generic formula to calculate the reactive power demand when the present power factor and the desired power factor are known. The same formula reduces to the following for desired power factor being Unity.

Where φ is the power factor angle.

We sincerely hope that this exercise helps you in calculating reactive power demand with more clarity.

Do you have a Challenging Power Factor Correction or Compensation issue, no matter how dynamic your load or reactive power is we can find the right solution for you. Get in touch with us today.

This article was originally published in

The author Mr. Hasan Mydin is part of the “Advanced Power Quality Solutions” department at InPhase. He is a self-confessed Power Quality enthusiast with more than a decade’s experience solving power quality problems across India and even abroad. To know more about him, check out  his LinkedIn Profile.

Meet you again with a interesting article, until then  bye from Dr.InPhase

Thank you!


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