Table of Contents
What Is kVA?
kVA (kilovolt-ampere) is the unit of apparent power in an AC electrical system. Apparent power is the product of voltage and current, representing the total power that must be delivered by the source regardless of how much is actually used to perform work. It combines both the real (active) power that does useful work and the reactive power that supports the electromagnetic fields in inductive and capacitive loads.
Understanding the distinction between kVA, kW, and kVAR is essential for electrical system design. Transformers and generators are rated in kVA because their capacity is limited by voltage and current, not by how much real work the load performs. A transformer rated at 100 kVA can deliver that amount of apparent power regardless of the load's power factor.
kVA Formulas
Power Triangle
| Power Type | Symbol | Unit | Description |
|---|---|---|---|
| Apparent Power | S | kVA | Total power delivered by source (V x I) |
| Real Power | P | kW | Power consumed doing useful work |
| Reactive Power | Q | kVAR | Power stored and returned by L/C elements |
The three quantities form a right triangle: S^2 = P^2 + Q^2. The power factor (PF) is the cosine of the angle between S and P, representing the fraction of apparent power that is actually doing work.
Generator & Transformer Sizing
- Always size equipment by kVA (apparent power), not kW (real power).
- A load of 85 kW at PF = 0.85 requires 100 kVA of transformer capacity.
- Low power factor increases required kVA for the same kW of real work.
- Power factor correction capacitors reduce reactive power and lower kVA requirements.
Frequently Asked Questions
What is the difference between kVA and kW?
kVA is apparent power (total power delivered), while kW is real power (power doing useful work). The relationship is kW = kVA x PF. For purely resistive loads (PF = 1), kVA equals kW. For inductive loads like motors (PF about 0.8), a 100 kVA source delivers only 80 kW of real power.
Why are generators rated in kVA not kW?
Generators are rated in kVA because the copper windings are limited by current (which determines heating) and the insulation is limited by voltage. The generator does not "know" the load's power factor, so its capacity is defined by the maximum voltage and current it can deliver, which is apparent power in kVA.
How do I convert kVA to amps?
For single phase: I = (kVA x 1000) / V. For three phase: I = (kVA x 1000) / (V x sqrt(3)). For example, a 100 kVA three-phase transformer at 480V draws 100,000 / (480 x 1.732) = 120.3 amps per line.