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kW vs kVA

kW vs kVA comparison: understand real power vs apparent power, power factor relationships, equipment sizing implications, and conversion formulas for transformers, generators, and electrical systems.

Enginist Team
Published: November 12, 2025
Updated: November 25, 2025

Table of Contents

kW vs kVA: Real Power vs Apparent Power Complete Guide

Quick AnswerWhat is the difference between kW and kVA?
kW (kilowatts) measures real power—the actual energy doing useful work. kVA (kilovolt-amperes) measures apparent power—the total power the source must supply. The relationship is **kW = kVA × Power Factor**. For typical motors (PF=0.85), a 100 kW load requires 118 kVA. Equipment is rated in kVA because heat losses depend on total current, not just useful work.

Quick Verdict

Understanding kW and kVA is fundamental to electrical system design. These aren't competing standards—they measure different aspects of electrical power that both matter for different purposes.

Bottom Line: Use kW when calculating energy consumption, equipment output, and electricity costs. Use kVA when sizing transformers, generators, UPS systems, and cables. Power factor connects them: kW = kVA × PF.

For most practical purposes: a 100 kW load requires 110-130 kVA of transformer capacity (depending on power factor), and a 100 kVA generator delivers only 80-85 kW of usable power.

At-a-Glance Comparison Table

FeaturekW (Kilowatts)kVA (Kilovolt-Amperes)Winner
What It MeasuresReal power (useful work)Apparent power (total supplied)—
SymbolPS—
Formula (AC)P=V×I×PFP = V \times I \times PFS=V×IS = V \times I—
Used ForEnergy bills, output ratingsEquipment sizing—
Utility BillingkWh energy chargesDemand charges (some utilities)—
Equipment RatingsMotor output, heater capacityTransformers, generators—
Relationship to CurrentDepends on PFDirectly proportional—
For Resistive LoadskW = kVAkVA = kWTie
For Inductive LoadskW < kVAkVA > kW—

Understanding Real Power (kW)

Real power is the actual energy that performs useful work in an electrical system. It's measured in kilowatts (kW) and represents:

  • Mechanical work: Running motors, pumps, compressors
  • Heat production: Electric heaters, ovens, process heating
  • Light production: All lighting systems
  • Electronic operation: Computers, controls, communications

Key Characteristics of kW

  • Appears on utility bills as kWh (energy consumption)
  • Motor output ratings are in kW (or horsepower: 1 HP ≈ 0.746 kW)
  • Represents actual work capability
  • What you pay for in energy charges

Real Power Formula

For AC single-phase:

P=V×I×cos⁡(ϕ)P = V \times I \times \cos(\phi)

For AC three-phase:

P=3×VLL×I×cos⁡(ϕ)P = \sqrt{3} \times V_{LL} \times I \times \cos(\phi)

Where cos⁡(ϕ)\cos(\phi) is the power factor.

Understanding Apparent Power (kVA)

Apparent power is the total power that must be supplied by the source, regardless of how much performs useful work. It's measured in kilovolt-amperes (kVA) and represents:

  • Total current flow: What conductors must carry
  • Equipment thermal loading: What causes heating in windings
  • Infrastructure capacity: What transformers and generators must deliver

Key Characteristics of kVA

  • Transformers, generators, UPS systems rated in kVA
  • Determines cable sizing and conductor ampacity
  • Always equal to or greater than kW
  • Some utilities charge demand based on kVA

Apparent Power Formula

For AC single-phase:

S=V×IS = V \times I

For AC three-phase:

S=3×VLL×IS = \sqrt{3} \times V_{LL} \times I

No power factor term—apparent power is simply voltage times current.

The Power Triangle: How kW and kVA Relate

The relationship between real power, reactive power, and apparent power forms a right triangle:

S2=P2+Q2S^2 = P^2 + Q^2

Or in practical units:

kVA2=kW2+kVAR2\text{kVA}^2 = \text{kW}^2 + \text{kVAR}^2

Where:

  • kW = Real power (horizontal leg)
  • kVAR = Reactive power (vertical leg)
  • kVA = Apparent power (hypotenuse)

Why Equipment is Rated in kVA

Transformers and generators are rated in kVA rather than kW because:

Thermal Limits Depend on Current

The heat generated in transformer windings follows:

Ploss=I2×RP_{loss} = I^2 \times R

This heating depends on total current magnitude, not power factor. A transformer carrying 100A generates the same heat whether that current delivers 80 kW (at 0.8 PF) or 100 kW (at 1.0 PF).

Universal Application

Rating in kVA allows the same transformer specification to apply regardless of load characteristics:

TransformerLoad PFAvailable kW
100 kVA1.0100 kW
100 kVA0.990 kW
100 kVA0.880 kW
100 kVA0.770 kW

The transformer operates safely at 100 kVA in all cases—it's the user's responsibility to account for their power factor.

Conversion Formulas

kW to kVA (Sizing Equipment)

When you know your load in kW and need to size equipment:

kVA=kWPF\text{kVA} = \frac{\text{kW}}{\text{PF}}
Transformer Sizing for 150 kW Load

Given:

  • Total load: 150 kW
  • Expected power factor: 0.85

Calculation:

kVA=1500.85=176.5 kVA\text{kVA} = \frac{150}{0.85} = 176.5 \text{ kVA}

With 25% safety margin:

kVArequired=176.5×1.25=220.6 kVA\text{kVA}_{required} = 176.5 \times 1.25 = 220.6 \text{ kVA}

Selection: 225 kVA transformer (next standard size)

kVA to kW (Understanding Capacity)

When you have equipment rated in kVA and need to know real power capacity:

kW=kVA×PF\text{kW} = \text{kVA} \times \text{PF}
Generator Real Power Capacity

Given:

  • Generator rating: 500 kVA
  • Rated power factor: 0.8

Calculation:

kW=500×0.8=400 kW\text{kW} = 500 \times 0.8 = 400 \text{ kW}

Result: This generator can supply up to 400 kW at its rated power factor. Loading it beyond 400 kW (even if apparent power is below 500 kVA) may exceed its prime mover capacity.

Power Factor Impact Analysis

Power factor dramatically affects the kVA requirement for a given kW load:

Power FactorkVA per 100 kW% Increase vs UnityTypical Load Type
1.00100 kVA0%Resistive heaters
0.95105 kVA5%Power factor corrected
0.90111 kVA11%Well-managed facility
0.85118 kVA18%Typical commercial
0.80125 kVA25%Industrial with motors
0.75133 kVA33%Heavy motor load
0.70143 kVA43%Uncorrected industrial

Practical Applications

Transformer Sizing

  1. Calculate total kW load with demand factors (NEC Article 220)
  2. Determine expected power factor
  3. Convert to kVA: kVA = kW / PF
  4. Add 20-25% safety margin
  5. Select next standard transformer size

Generator Selection

Generators often have both ratings:

  • kVA rating: Maximum apparent power
  • kW rating: Maximum real power at rated PF

The limit reached first governs. A "100 kVA / 80 kW" generator at 0.8 PF:

  • Can supply 100 kVA maximum
  • Can supply 80 kW maximum
  • At 0.9 PF: limited to 80 kW (not 90 kW)
  • At 0.7 PF: limited to 70 kW by kVA (not 80 kW)

UPS System Selection

UPS systems are typically kVA-rated:

  1. Sum all load kW
  2. Estimate or measure power factor
  3. Calculate kVA = kW / PF
  4. Add 20-30% margin for future growth
  5. Select appropriate UPS rating

Common Mistakes to Avoid

MistakeImpactPrevention
Assuming kW = kVAUndersized equipmentAlways account for power factor
Using wrong PFSizing errorsMeasure or use conservative estimates
Ignoring PF variationOverload at low PFUse worst-case (lowest) expected PF
No safety marginNo growth capacityAdd 20-25% to calculations
Confusing ratingsWrong equipmentCheck both kW and kVA limits

Related Tools

Use these calculators for kW and kVA conversions:

Key Takeaways

  • kW is real power that does useful work; kVA is apparent power the source must supply
  • Conversion formulas: kW = kVA × PF and kVA = kW / PF
  • Equipment sizing: Use kVA for transformers, generators, UPS—they're rated for total current, not just useful power
  • Power factor impact: Poor PF (0.70-0.80) requires 25-43% more kVA than good PF (0.95)
  • Both matter: kW for energy/costs, kVA for infrastructure sizing

Further Reading

References & Standards

  • IEC 60050-131: International definitions for kW, kVA, power factor
  • IEEE 141: Recommended practice for industrial power systems
  • IEEE C57.12.00: Transformer rating standards
  • NEC Article 220: Load calculations and demand factors

Disclaimer: This comparison provides general technical guidance based on international standards. Equipment sizing should be verified with specific manufacturer data and local code requirements. Always consult licensed electrical engineers for critical installations.

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