What is Power Factor? Complete Guide with Formula, Types, Improvement Methods and Practical Examples
Power factor is one of the most important concepts in electrical engineering and electrical systems. Whether you are an electrician, student, technician, or preparing for competitive exams, understanding power factor is essential. It directly affects electricity efficiency, power consumption, energy bills, and system performance.
In this detailed guide, you will learn what power factor is, its formula, types of power factor, causes of low power factor, disadvantages, improvement methods, capacitor calculations, industrial applications, and practical examples — explained in simple English.
What is Power Factor?
Power factor is the ratio of real power (working power) to apparent power in an electrical system. It tells us how effectively the electrical power is being converted into useful work output.
In simple words, power factor shows how efficiently electrical power is used. A higher power factor means better efficiency. A lower power factor means more wasted power.
Power Factor = Real Power ÷ Apparent Power
- Real Power = kW (kilowatts)
- Apparent Power = kVA (kilovolt-amperes)
- Power Factor has no unit (it is a ratio)
Power Factor Formula
Power factor is also defined using the phase angle between voltage and current.
Power Factor = cos φ (cosine of phase angle)
Where φ (phi) is the angle between voltage and current waveforms.
- If voltage and current are in phase → cos φ = 1 → Power factor = 1
- If current lags or leads voltage → cos φ < 1 → Power factor less than 1
Understanding Power Factor with Simple Example
Suppose a machine uses:
- Real Power = 80 kW
- Apparent Power = 100 kVA
Power Factor = 80 / 100 = 0.8
This means only 80% of supplied power is doing useful work. The remaining power is circulating in the system.
Types of Power Factor
1. Unity Power Factor
When voltage and current are perfectly in phase, the power factor is 1 (or unity). This is the best and most efficient condition.
Examples: Pure resistive loads like heaters, incandescent lamps.
2. Lagging Power Factor
When current lags behind voltage, it is called lagging power factor. This occurs in inductive loads.
Examples:
- Motors
- Transformers
- Induction coils
- Chokes
3. Leading Power Factor
When current leads voltage, it is called leading power factor. This happens in capacitive loads.
Examples:
- Capacitor banks
- Over-corrected PF systems
- Some electronic circuits
Why Power Factor is Important
Power factor is very important for both electricity consumers and electricity suppliers. A poor power factor increases system losses and electricity cost.
Benefits of High Power Factor
- Lower electricity bills
- Reduced line losses
- Better voltage regulation
- Higher system efficiency
- Smaller conductor size required
- Improved equipment life
Disadvantages of Low Power Factor
Low power factor creates many technical and financial problems.
- Higher current flow in the system
- Increased copper losses (I²R losses)
- Larger cable size required
- Voltage drop increases
- Reduced transformer capacity
- Penalty charges from electricity board
- Overheating of equipment
Main Causes of Low Power Factor
1. Induction Motors
Induction motors are the biggest cause of low power factor because they require magnetizing current.
2. Transformers
Transformers draw magnetizing current even at no load.
3. Fluorescent Lighting
Choke-based lighting systems create lagging PF.
4. Welding Machines
Arc welding equipment is highly inductive.
5. Lightly Loaded Motors
Motors running at low load have poor power factor.
Effects of Power Factor on Electricity Bill
Many electricity boards charge penalties for low power factor (below 0.9 or 0.95). Industries must maintain a good PF to avoid penalty.
Low PF means:
- More kVA demand
- Higher demand charges
- Extra billing penalties
Power Triangle Explanation
Power factor is best understood using the power triangle.
- Horizontal side = Real Power (kW)
- Vertical side = Reactive Power (kVAR)
- Hypotenuse = Apparent Power (kVA)
Power Factor = kW / kVA = cos φ
Methods to Improve Power Factor
1. Capacitor Banks
Capacitors supply leading reactive power and cancel lagging reactive power. This is the most common PF correction method.
- Installed near load
- Automatic PF panels used in industries
- Low cost and effective
2. Synchronous Condenser
An over-excited synchronous motor acts as a capacitor and improves PF.
3. Phase Advancer
Used with induction motors to improve PF by supplying rotor excitation.
Capacitor Size Calculation for Power Factor Improvement
Required kVAR = kW × (tan φ₁ − tan φ₂)
- φ₁ = initial PF angle
- φ₂ = required PF angle
Example:
Load = 100 kW
Initial PF = 0.7
Target PF = 0.95
Calculate capacitor kVAR needed using formula.
Power Factor in Single Phase System
Single-phase PF = cos φ between voltage and current.
Measured using:
- Wattmeter
- Voltmeter
- Ammeter
Power Factor in Three Phase System
In three-phase systems:
PF = kW / (√3 × V × I)
Industries monitor three-phase PF continuously using PF meters.
Typical Power Factor Values
| Equipment | Power Factor |
|---|---|
| Incandescent Lamp | 1.0 |
| Induction Motor (Full Load) | 0.8 – 0.9 |
| Induction Motor (Light Load) | 0.2 – 0.5 |
| Transformer | 0.7 – 0.9 |
| Fluorescent Lamp | 0.5 – 0.6 |
Industrial Power Factor Correction Panels
Automatic Power Factor Correction (APFC) panels automatically switch capacitor banks based on load conditions to maintain PF above 0.95.
- Controller based switching
- Step capacitors
- Relay or thyristor switching
Difference Between Real, Reactive and Apparent Power
- Real Power (kW): Does useful work
- Reactive Power (kVAR): Supports magnetic field
- Apparent Power (kVA): Total supplied power
Power Factor Meter
Power factor meters measure PF directly. Types include:
- Analog PF meter
- Digital PF meter
- Smart energy meter
- Panel mounted PF meter
Frequently Asked Questions (FAQs)
What is a good power factor?
0.95 to 1.0 is considered good.
Can power factor be more than 1?
No. PF cannot exceed 1.
Why is PF usually lagging?
Because most loads are inductive.
Does power factor affect home users?
Usually no direct billing effect, but affects efficiency.
Which device improves power factor?
Capacitor bank.
Conclusion
Power factor is a key parameter in electrical systems that indicates how efficiently power is used. Maintaining a high power factor reduces losses, improves system capacity, and lowers electricity cost. Industries widely use capacitor banks and automatic PF correction panels to maintain high PF levels.
Understanding power factor is essential for electricians, students, and engineers. With proper PF correction methods, electrical systems can operate more efficiently and economically.