About the kVA Calculator
kVA (kilovolt-amperes) measures apparent power — the combined effect of voltage and current flowing through an AC circuit, regardless of how much of that power actually does useful work. It is the standard rating used for generators, transformers, and UPS systems, because that equipment must be sized for the full current it carries, not just the real power delivered. This calculator finds apparent power (kVA), real power (kW), reactive power (kVAR), and the phase angle from voltage, current, and power factor, for both single-phase and three-phase circuits.
Understanding the formula
For a single-phase circuit, apparent power is simply voltage times current: kVA = (V × I) / 1000, where V is in volts and I is in amps (dividing by 1000 converts volt-amps to kilovolt-amps). For a balanced three-phase circuit, kVA = (√3 × V × I) / 1000, where V is the line-to-line voltage and √3 ≈ 1.7321 accounts for the 120° phase offset between the three legs. Apparent power, real power, and reactive power form a right triangle — the "power triangle" — where kVA is the hypotenuse: (kVA)² = (kW)² + (kVAR)². Real power is kW = kVA × PF, and reactive power is kVAR = kVA × sin(θ), where θ = cos⁻¹(PF) is the phase angle between voltage and current.
Single-phase vs. three-phase circuits
Residential circuits and small appliances are almost always single-phase. Industrial equipment, large motors, and commercial buildings typically run on three-phase power because it delivers more power per conductor and produces smoother torque in motors. The most common mistake when sizing three-phase kVA is entering line-to-neutral voltage instead of line-to-line voltage — line-to-line voltage is √3 times larger (for example, a 230 V line-to-neutral system corresponds to about 400 V line-to-line), so using the wrong one misstates the result by a factor of √3.
Why kVA matters for equipment sizing
Generators, transformers, and UPS units are rated in kVA rather than kW because their windings and conductors must handle the full current flowing through them, regardless of power factor. A load with a low power factor draws more current for the same real power, so equipment sized only in kW could be undersized. As a rule of thumb, size backup power and transformers with 20-25% headroom above the calculated kVA to allow for inrush currents, future load growth, and non-unity power factor loads like motors and electronics.