kVA Calculator

The kVA Calculator computes the apparent power in kilovolt-amperes (kVA) from voltage and current for three-phase electrical systems, and shows the equivalent kW at common power factor values. kVA is the fundamental rating unit for generators, transformers, UPS systems, and switchgear, making this one of the most frequently used calculations in electrical engineering.

For three-phase systems, apparent power S (kVA) = √3 × V_L × I_L / 1000, where V_L is line-to-line voltage and I_L is line current. The √3 factor (≈1.732) accounts for the phase relationship between three voltage vectors 120° apart. For single-phase systems, S = V × I / 1000 without the √3 factor.

The distinction between kVA and kW matters enormously in electrical design. A generator rated 1000 kVA can supply a maximum of 800 kW at 0.8 PF, or 900 kW at 0.9 PF, or 1000 kW at unity PF. If your load is 900 kW at 0.8 PF, you need 900/0.8 = 1125 kVA of generation capacity — not just 900 kVA. Undersizing generators for kVA is a common and costly mistake.

Similarly, transformer ratings in kVA represent the maximum continuous current-handling capacity. The transformer windings heat based on current (I²R), not active power. A 500 kVA transformer supplying 400 kW at 0.8 PF is fully loaded (400/0.8 = 500 kVA). It cannot supply an additional 100 kW of purely resistive load (unity PF) without overloading.

UPS systems are universally rated in kVA because their inverters and battery systems must supply the full volt-ampere product. Battery sizing, inverter current rating, and bypass switch sizing all depend on kVA, not kW. Power factors of typical UPS loads vary widely — servers and networking equipment typically 0.9-0.99 PF, while older computer equipment may be 0.6-0.7 PF.