How to Calculate Power Factor: A Step-by-Step

By R.W. Hurst, Editor

How to calculate power factor involves dividing real power (kW) by apparent power (kVA). This simple formula helps businesses assess electrical efficiency, reduce waste, and optimize equipment performance for lower energy costs.

Understanding how to calculate power factor (PF) is essential for improving electrical system performance. By measuring the ratio of real power to apparent power (S), facility managers can pinpoint inefficiencies, identify equipment issues, and take corrective actions to minimize energy losses. Proper calculation ensures smoother operations, fewer penalties from utilities, and significant cost savings over time.

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How to Calculate Power Factor

Knowing how to calculate PF is essential for understanding the efficiency of an electrical system. The calculation is simple once you know two key values: real power and S.

The Basic Formula

The standard formula for PF is:

PF = Real Power (kW) ÷ Apparent Power (S)

Real power is the energy actually consumed by equipment to perform work. S is the total energy supplied, combining both usable and non-usable components.

Step-by-Step Guide

Measure Real Power (kW):
Use a wattmeter to determine the real energy consumption.
Measure S (kVA):
Measure voltage (V) and current (A), then calculate kVA using the formula:

S (kVA) = (Voltage × Current) ÷ 1000
Apply the PF Formula:
Divide real power by S.

Practical Example

Suppose you measure the following in your system:

Real Power = 300 kW
Voltage = 400 V
Current = 500 A

First, calculate S:

(400 × 500) ÷ 1000 = 200 kVA

Now calculate PF:

300 ÷ 200 = 1.5 (which indicates an error; you cannot have a PF above 1)

Let's correct this:
(400 × 500) ÷ (1000 × √3) ≈ 346 kVA (for a 3-phase system)

Then:

300 ÷ 346 ≈ 0.87

Result: The PF is 0.87, meaning the system operates at 87% efficiency.

Need a faster way? Use our Power Factor Calculator to automate the calculation.

Why It Matters

Accurately calculating the power factor enables facilities to identify inefficiencies and correct them before they result in high energy costs or penalties.
Learn about techniques in Power Factor Correction.

Additional Tips

Always measure during peak load times for accuracy.
Use reliable measuring devices, such as clamp meters and digital analyzers.
If your calculated PF is below 0.9, improvements such as capacitor banks or Automatic Power Factor Controllers may be necessary.

For further reading on What is Power Factor? or how Capacitors Help Improve Efficiency, see our detailed guides.