Kva Calculator

What the kVA calculator does

kVA is the unit of apparent power, the quantity used to size transformers, generators, and electrical supplies. This calculator finds it from voltage and current, for single-phase and three-phase systems, with the line-to-line and line-to-neutral options that three-phase work needs.

Below is what apparent power is, the equation behind it, how it differs from real power in kilowatts, and a worked example.

How to use it

1. Choose the load distribution: single-phase or three-phase.
2. Enter the voltage and current, selecting line-to-line or line-to-neutral voltage for three-phase.
3. Press Calculate for the apparent power in kVA, or Reset to clear it.

What kVA and apparent power are

kVA stands for kilovolt-amperes, the unit of apparent power in an electrical system. Apparent power is the product of the voltage and the current, the total power that appears to flow regardless of how much of it does useful work. It is the quantity that the wiring, transformers, and supply equipment must actually handle, because they have to carry the full voltage and the full current whether or not the two are perfectly in step. A kilovolt-ampere is simply a thousand volt-amperes, a convenient size for the power levels of real installations.

Apparent power matters because electrical equipment is limited by voltage and current, not directly by useful power. A cable can carry only so much current, and a transformer can handle only so much voltage and current together, so their capacity is naturally expressed in volt-amperes. This is why the kVA figure, rather than the useful-power figure, is what you use to size supply equipment. This calculator computes it from the voltage and current of single-phase or three-phase systems, giving the number that sizing decisions depend on.

The equation it uses

For a single-phase system, apparent power is simply voltage times current:

S = V × I

expressed in kilovolt-amperes by dividing by a thousand. For a three-phase system measured line to line, a factor involving the square root of three enters, S = √3 × V × I, reflecting how the three phases combine. Measured line to neutral instead, the three phases each contribute, giving S = 3 × V × I. The calculator applies the right form for the system and voltage type you select, so the apparent power comes out correctly whether you are working on a simple single-phase circuit or a three-phase installation.

kVA versus kW

One of the most common points of confusion in electrical work is the difference between kVA and kW, between apparent power and real power. Real power, in kilowatts, is the power that actually does useful work, producing heat, light, or motion. Apparent power, in kilovolt-amperes, is the total power that flows, which can be larger because in many alternating-current circuits the voltage and current are not perfectly in step. The two are equal only when they are in step, as with a simple resistive load.

The link between them is the power factor, the fraction of the apparent power that is real. A circuit with a power factor of one has its kW equal to its kVA, while a lower power factor means the kVA exceeds the kW, with the difference made up by power that sloshes back and forth without doing net work. This is why both numbers exist and why they are not interchangeable. The kVA figure tells you the load on the equipment; the kW figure tells you the useful output. This calculator gives the apparent power in kVA, the quantity for sizing, and the power factor calculator handles the relationship to real power.

Why equipment is rated in kVA

Transformers and generators are almost always rated in kVA rather than kW, and the reason comes straight from how they are limited. A transformer's windings can carry only so much current before they overheat, and they are designed for a certain voltage, so the product of voltage and current, the apparent power, is the true ceiling on what the transformer can deliver. This ceiling does not depend on the power factor of the load, so it is naturally expressed in volt-amperes.

If such equipment were rated in kilowatts instead, the rating would be misleading, because a load with a poor power factor would draw more current, and so more apparent power, than its useful-power figure suggests, potentially overloading the equipment even though the kilowatts looked safe. Rating in kVA avoids this trap by stating the limit in the terms that actually matter to the hardware. This is why sizing a transformer or generator for a load means working in kVA, and why this calculator gives you that figure directly from the voltage and current.

Units and precision

The calculator takes voltage in volts, kilovolts, or millivolts and current in amperes, milliamperes, or kiloamperes, and returns the apparent power in kilovolt-amperes. It applies the correct relationship for single-phase or three-phase systems, including the square-root-of-three factor and the choice of line-to-line or line-to-neutral voltage. The calculation is exact; the result is the apparent power used to size supply equipment.

A worked example

Suppose a single-phase load runs at 240 volts and draws 20 amperes.

The apparent power is S = V × I = 240 × 20 = 4,800 volt-amperes, or 4.8 kVA. For a three-phase example, a load at 400 volts line to line drawing 30 amperes has an apparent power of √3 × 400 × 30 = about 20,800 volt-amperes, or 20.78 kVA, with the square-root-of-three factor reflecting the three-phase supply. These are the figures you would use to size the transformer or generator feeding each load.

Questions people ask

How do you calculate kVA?

For single-phase, multiply voltage by current and divide by a thousand. For three-phase line to line, multiply by the square root of three as well: S = √3 × V × I.

What is the difference between kVA and kW?

kVA is apparent power, the total that flows; kW is real power, the useful part. They are equal only at a power factor of one; otherwise kVA is larger.

Why are transformers rated in kVA?

Because they are limited by voltage and current, whose product is apparent power, regardless of the load's power factor. Rating in kVA states the true ceiling on the hardware.

How do I convert kVA to kW?

Multiply the kVA by the power factor: kW = kVA × PF. A 10 kVA load at a power factor of 0.8 delivers 8 kW of useful power.

References

A quick note on where the physics comes from. Apparent power, real power, and three-phase relationships are standard physics and electrical engineering, set out in OpenStax's University Physics and in Georgia State University's HyperPhysics. The units follow NIST. The HyperPhysics link is worth a quick click to confirm it lands where you expect.

1. OpenStax, University Physics Volume 2, Section 15.4, Power in an AC Circuit. https://openstax.org/books/university-physics-volume-2/pages/15-4-power-in-an-ac-circuit
2. HyperPhysics, AC Power. http://hyperphysics.phy-astr.gsu.edu/hbase/electric/powerac.html
3. National Institute of Standards and Technology (NIST), SP 811, Guide for the Use of the International System of Units. https://www.nist.gov/pml/special-publication-811