# KVA Calculator

Calculate kVA from voltage and current for single-phase and three-phase systems. Convert between kVA, kW, and kVAR with power factor.

## What this calculates

KVA (kilovolt-amperes) measures the apparent power in an electrical system. It is what transformers, generators, and UPS systems are rated in because they must handle the full current regardless of the power factor. A single-phase 240V circuit drawing 50 amps has an apparent power of 12 kVA. This calculator also converts to real power (kW) and reactive power (kVAR).

## Inputs

- **Voltage** (V) — min 0 — Supply voltage (line-to-line for three-phase).
- **Current** (A) — min 0 — Current draw in amperes.
- **Phase** — options: Single Phase, Three Phase — Single-phase or three-phase power system.
- **Power Factor** — min 0.1, max 1 — Power factor for kW calculation (0 to 1).

## Outputs

- **Apparent Power** (kVA) — Apparent power in kilovolt-amperes.
- **Real Power** (kW) — Real (active) power in kilowatts.
- **Reactive Power** (kVAR) — Reactive power in kilovolt-amperes reactive.
- **Full Load Amps** (A) — Current at the given voltage and kVA.

## Details

**The formulas:**

- **Single-phase:** kVA = V × I / 1,000
- **Three-phase:** kVA = √3 × V × I / 1,000

**kVA vs. kW vs. kVAR:**

These three form a power triangle:
- **kVA (apparent power):** The total power the system must deliver, including both useful and wasted components. This is what sizes your transformer or generator.
- **kW (real power):** The portion that does actual work (lights, heat, motion). kW = kVA × power factor.
- **kVAR (reactive power):** The portion that shuttles back and forth between the source and inductive/capacitive loads. It does not do useful work but still requires current capacity.

**Why kVA matters for equipment sizing:**

A generator rated at 100 kW with a power factor of 0.8 actually needs to be a 125 kVA unit. If you size it at 100 kVA, the generator will be overloaded even though the real power draw is within its kW rating. This is because the generator's windings and cooling system must handle the full current, not just the real power component.

**Common equipment ratings:**
| Equipment | Typical Rating |
|---|---|
| Residential transformer | 25-50 kVA |
| Small commercial transformer | 75-500 kVA |
| Standby generator (home) | 10-20 kW / 12-25 kVA |
| UPS for server rack | 3-10 kVA |
| Industrial motor starter | 50-500 kVA |

## Frequently Asked Questions

**Q: What is the difference between kVA and kW?**

A: kVA is apparent power (total power the system must deliver), and kW is real power (the portion that does useful work). They are related by the power factor: kW = kVA x power factor. At a power factor of 1.0, kVA equals kW. At a power factor of 0.8, you need 125 kVA to deliver 100 kW.

**Q: How do I size a transformer using kVA?**

A: Add up the kVA (not kW) of all loads the transformer will serve. Then add a safety margin of 20-25%. For example, if your total load is 80 kVA, choose a standard 100 kVA transformer. Undersizing based on kW instead of kVA is a common and costly mistake.

**Q: Why are generators rated in both kW and kVA?**

A: The kW rating tells you the real power output at a specific power factor (usually 0.8). The kVA rating tells you the maximum apparent power the alternator can handle. A 100 kW / 125 kVA generator can deliver 100 kW at 0.8 power factor, or less kW at a lower power factor, but never more than 125 kVA regardless.

**Q: How do I convert kVA to amps?**

A: For single-phase: Amps = kVA x 1,000 / Voltage. For three-phase: Amps = kVA x 1,000 / (sqrt(3) x Voltage). For example, a 50 kVA single-phase load at 240V draws 208 amps. The same 50 kVA as a three-phase load at 480V draws only 60 amps.

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Source: https://vastcalc.com/calculators/physics/kva
Category: Physics
Last updated: 2026-04-08