Introduction #

kW (kilowatts) is real power—the rate at which energy is converted into useful work. kVA (kilovolt-amperes) is apparent power—the product of voltage and current that the supply must deliver. In engineering you must distinguish them: equipment is sized in kVA, while billing and energy use are often in kW. Confusing the two leads to undersized transformers, wrong generator selection, and incorrect cost estimates. This guide defines both terms, gives the formulas that link them, and shows when to use each.

Definitions #

Real Power (kW) #

Real power is the component of power that does useful work: shaft output of motors, heat from heaters, light from lamps. It is measured in kilowatts (kW). Utility energy charges and most demand charges are based on kW. 1 kW = 1,000 watts.

Apparent Power (kVA) #

Apparent power is the product of voltage and current (in magnitude). It is measured in kilovolt-amperes (kVA). Transformers, generators, and UPS are rated in kVA because they must supply the total current, not just the in-phase component. kVA = (Voltage × Current) ÷ 1,000 for single-phase; for three-phase the standard formula includes √3.

Reactive Power (kVAR) #

Reactive power (kVAR) is the power that oscillates between source and load to build magnetic and electric fields. It does not do net work over a cycle. Motors and transformers need reactive power to operate. It increases the current for a given real power and thus the kVA.

Power Factor #

Power factor (PF) is the ratio of real power to apparent power:

PF = kW ÷ kVA

So:

kW = kVA × PF
kVA = kW ÷ PF

The relationship between real, reactive, and apparent power is:

kVA = √(kW² + kVAR²)

or equivalently kVA² = kW² + kVAR². PF ranges from 0 to 1.0. Resistive loads have PF = 1.0; typical industrial motor loads are 0.8–0.95. Low PF means higher kVA for the same kW and often triggers utility penalties.

Visual Comparison: kW vs kVA #

Parameter kW kVA
Represents Real power Apparent power
Includes reactive power? No Yes
Used for billing? Yes Sometimes
Used for equipment sizing? No Yes

Engineering Examples #

Motor Example #

A 50 kW motor runs at 0.85 power factor. What kVA must the supply deliver?

kVA = kW ÷ PF = 50 ÷ 0.85 = 58.8 kVA

The system must supply 58.8 kVA to deliver 50 kW of real power. Sizing cables or breakers from the 50 kW alone would underestimate the current.

Generator Sizing Example #

Emergency backup for 200 kW critical load at 0.80 PF and 50 kW essential load at 0.90 PF. Find the required generator kVA.

Critical kVA = 200 ÷ 0.80 = 250 kVA
Essential kVA = 50 ÷ 0.90 = 55.6 kVA
Total kVA = 250 + 55.6 = 305.6 kVA

Select a 350 kVA generator (with margin for starting currents). A generator rated only 200 kVA would be overloaded.

Transformer Sizing Example #

A factory has 300 kW motors (PF 0.85), 50 kW lighting (PF 1.0), and 20 kW office equipment (PF 0.90). Size the transformer.

Motor kVA = 300 ÷ 0.85 = 352.9 kVA
Lighting kVA = 50 ÷ 1.0 = 50 kVA
Office kVA = 20 ÷ 0.90 = 22.2 kVA
Total kVA = 352.9 + 50 + 22.2 = 425.1 kVA

Select a 500 kVA transformer (next standard size with margin). Transformers are rated in kVA; using 370 kW (300+50+20) would understate the required kVA.

When to Use kW vs kVA #

Utility billing: Many tariffs use kW for energy and often for demand. Some use kVA demand to penalize low power factor. Check your tariff; in both cases you need to know kW and PF to interpret kVA.

Generator selection: Generators are rated in kVA. Use the load’s kW and PF to get required kVA (kVA = kW ÷ PF), then add margin for starting and growth.

UPS sizing: UPS capacity is in kVA (and sometimes kW). For a given load kW and PF, kVA = kW ÷ PF. Size the UPS to the total kVA of the load with appropriate margin.

Transformer rating: Transformers are kVA-rated. Sum the kVA of all loads (each load kW ÷ its PF) or use total kW and a weighted PF to get total kVA, then choose the next standard size with margin.

For quick conversions, use our kW to kVA converter or calculate kVA from kW with your power factor. For more on PF and correction, see the Power Factor Guide.

Related articles: Power Factor Correction: Best Practices; How to Calculate Transformer Size; How to Calculate Factory Load.

Frequently Asked Questions #

Is kVA always higher than kW? #

Yes, for a given load. kVA is the hypotenuse of the power triangle; kW = kVA × PF, and PF ≤ 1. So kW ≤ kVA. They are equal only at unity power factor (PF = 1.0).

Why do generators use kVA? #

Generators must supply the total current (real + reactive). Their limits are set by current and voltage, i.e. apparent power (kVA). Rating in kVA tells you the maximum kVA they can deliver; the actual kW depends on the load’s power factor.

Can kW equal kVA? #

Yes, when power factor is 1.0. Then kW = kVA. That happens for purely resistive loads (heaters, incandescent lighting). Most industrial loads have PF < 1, so kW < kVA.

What happens at PF 0.8? #

At 0.8 power factor, kVA = kW ÷ 0.8 = 1.25 × kW. So for 100 kW you need 125 kVA. The extra 25 kVA is reactive; it increases current and losses but not useful work. Many utilities charge more when PF is below 0.9–0.95.

Why is power factor important? #

Low power factor increases current for the same kW, so you need larger cables, breakers, transformers, and generators. Utilities often charge demand or penalty fees for low PF. Improving power factor (e.g. with capacitors) reduces kVA and can lower costs and allow use of smaller equipment.

Does improving power factor reduce energy consumption (kW)? #

No. Power factor correction reduces kVA and current, and often reduces demand charges and losses. It does not change the real power (kW) consumed by the load for the same mechanical or output work.

What is a good power factor for industrial facilities? #

Most utilities expect PF ≥ 0.90 to avoid penalties. Target 0.92–0.95 for a balance of cost and equipment stress. Below 0.85 often triggers surcharges.

Conclusion #

kW is real power; kVA is apparent power. They are related by power factor: kW = kVA × PF and kVA = √(kW² + kVAR²). Use kW for energy and billing analysis; use kVA for sizing transformers, generators, UPS, and conductors. Always account for power factor when converting between kW and kVA. Use our kW to kVA converter for quick calculations.