Who This Guide Is For
Use this guide when you need to understand why equipment ratings, utility bills, generators, UPS systems, and transformers may use kVA even when the load is described in kW.
Three Types of AC Power
| Quantity | Unit | Meaning |
|---|---|---|
| Real power | kW | Average power converted to work, heat, light, or another useful output. |
| Reactive power | kVAR | Power exchanged with magnetic and electric fields. |
| Apparent power | kVA | Combined voltage-current burden on the electrical system. |
For a sinusoidal load, these quantities form a power triangle:
kVA² = kW² + kVAR²
Formula Summary
Power factor is the ratio of real power to apparent power:
PF = kW ÷ kVA
Rearranging gives:
kW = kVA × PF kVA = kW ÷ PF
At unity power factor, kW and kVA are numerically equal. At lower power factor, more kVA and current are required for the same kW.
Worked Example
A load uses 80 kW at 0.80 power factor:
kVA = 80 kW ÷ 0.80 = 100 kVA
The source and conductors carry 100 kVA even though only 80 kW is converted to average useful power. The corresponding reactive power is:
kVAR = √(100² - 80²) = 60 kVAR
Why Low Power Factor Matters
For fixed real power and voltage, lower power factor means higher current. Higher current increases conductor losses, voltage drop, and capacity used in transformers and generators. Some utilities apply penalties or demand charges when large customers operate below a required power factor.
Capacitor banks can reduce inductive reactive power, but correction must account for harmonics, resonance, switching, and changing load.
Why Some Equipment Is Rated in kVA
Transformers, generators, and UPS systems often have kVA limits because winding and conductor heating depend strongly on voltage and current. They may also have a separate kW limit imposed by the engine, inverter, thermal design, or energy source.
A load must fit within all applicable kW, kVA, current, transient, harmonic, and environmental limits.
Common Mistakes
- Treating kW and kVA as identical when power factor is below 1.0.
- Comparing load kW directly to transformer or generator kVA without conversion.
- Ignoring harmonic distortion when true power factor matters.
- Assuming capacitor correction reduces real power consumption.
Reference Notes and Assumptions
The power triangle is a standard sinusoidal AC relationship. Real systems with harmonics, variable-frequency drives, or nonlinear loads may require true power factor and measured waveform data.