KVA Calculator - Apparent, Real, and Reactive Power

A kVA calculator turns RMS voltage and current into apparent power in kilovolt-amperes, then splits that apparent power into real kilowatts and reactive kVAr through the power factor. It accepts single-phase and three-phase inputs because the same answer needs different multipliers, and reports the result in kVA, kW, and kVAr on one screen.

• • Sizing a UPS or generator for a known load: Convert a nameplate current and voltage into the kVA figure that a UPS spec sheet quotes, then read off the kW the load will actually draw.
• • Working with three-phase industrial mains: Convert 400 V line-to-line or 230 V line-to-neutral industrial measurements into apparent kVA and real kW for motors and panel feeds.
• • Estimating power-factor penalties: Compare kW at the measured PF with kVA to see how much apparent power the chosen PF leaves on the table.
• • Translating clamp-meter readings: Turn a clamp-meter current reading at a known supply voltage into kVA, kW, and kVAr without re-deriving the current type.

AC power differs from DC because the voltage and current oscillate and may not peak together. The wiring still has to carry the full apparent power in kVA even when only the real power in kW does useful work. The kVA calculator makes that split visible by computing kVA first and then kW and kVAr from the power factor.

When you need the same answer in watts, volt-amperes, and var, AC Wattage Calculator reports the wattage, VA, and var for the same single-phase and three-phase inputs.

The calculator picks a phase factor k from the current-type selector, multiplies k by the RMS voltage and current to get apparent power S in volt-amperes, then divides S by 1000 to convert it to kVA. It also multiplies S by the power factor to get real power P in kW and computes reactive power Q in kVAr from the sqrt(1 - PF^2) share of S.

• V: RMS voltage in volts. Use V_LL for three-phase line-to-line or V_LN for three-phase line-to-neutral.
• I: RMS current the AC load draws, in amperes, from a clamp meter or nameplate.
• PF: Power factor, the cosine of the phase angle between voltage and current. 1.0 for a pure resistor, 0.8 to 0.95 for typical induction motors.
• k: Phase factor from the current-type selector: 1 for single-phase, sqrt(3) for three-phase line-to-line, 3 for three-phase line-to-neutral.
• S: Apparent power in kVA, equal to k * V * I / 1000. Sets wiring, transformer, generator, and UPS sizing.
• P: Real power in kW, equal to S * PF. The number used for energy billing and useful work.
• Q: Reactive power in kVAr, equal to S * sqrt(1 - PF^2). Oscillates between source and load and is reduced by PF correction.

The three apparent-power formulas share the shape S_kVA = k * V * I / 1000, with k = 1, sqrt(3), or 3. S is what the wiring carries, P is what gets billed, and Q is what PF correction reduces.

According to Wikipedia - AC Power, the apparent power in an AC circuit equals the product of RMS voltage and RMS current, and the real power is the apparent power multiplied by the cosine of the phase angle.

According to Wikipedia - Three-Phase Electric Power, balanced three-phase apparent power is S = sqrt(3) * V_LL * I and the corresponding real power is P = sqrt(3) * V_LL * I * cos(phi).

When the power factor itself is the unknown and you have to back it out from real kW and apparent kVA, Power Factor Calculator returns PF, the phase angle, and the reactive kVAr from the same two inputs you see here.

Four ideas cover most apparent-power problems in physics, motor sizing, and UPS selection.

These four ideas carry over to any apparent-power problem, from UPS selection to power-quality work.

When the next step is to convert the kW into mechanical work over a measured time interval, Work Energy Power Calculator carries the same kilowatts through the work-energy-power formulas.

Five steps from a measured voltage, current, and power factor to a complete apparent-power answer in kVA, kW, and kVAr.

1. 1 Pick the current type: Choose single-phase for typical 120 V or 240 V household circuits, three-phase line-to-line for industrial 400 V mains, or three-phase line-to-neutral for phase voltage only.
2. 2 Enter the RMS voltage: Type the RMS voltage from the meter or nameplate. Use V_LL for three-phase line-to-line or V_LN for three-phase line-to-neutral.
3. 3 Enter the RMS current: Type the RMS current the load draws, in amperes, from a clamp-meter reading on the supply side.
4. 4 Enter the power factor: Pick a power factor between 0 and 1. Use 1.0 for resistive loads, 0.85 to 0.95 for typical induction motors at full load.
5. 5 Read the results: The result panel shows kVA, kW, kVAr, the power factor used, the percentage of apparent power lost to the power factor, the phase angle in degrees, and the phase factor k.

Practical example: a 240 V single-phase workshop circuit draws 20 A at 0.8 PF. Type 240 V, 20 A, and PF 0.80, and the calculator returns 4.8 kVA, 3.84 kW, 2.88 kVAr, and 20% power-factor loss.

When sizing branch circuits from the apparent-power result, Electrical Load Calculator returns the running and starting load in amps from the same voltage and power factor.

An apparent-power workflow catches the cases where multiplying volts by amps would quote a real-power number the wiring cannot deliver.

• • Three current-type modes in one tool: Switch between single-phase, three-phase line-to-line, and three-phase line-to-neutral without retyping values.
• • Apparent, real, and reactive power in one screen: kVA, kW, and kVAr are computed side by side, so the power factor role in UPS and generator sizing is visible without switching pages.
• • Output units match industry spec sheets: Generator and UPS manufacturers quote capacity in kVA, so the result lines up directly with the rating plate instead of forcing a unit conversion.
• • Input validation with status notes: Out-of-range power factors, zero inputs, and unusual voltage combinations surface clear status notes.
• • Cross-validation-friendly defaults: The default 120 V / 10 A / 0.85 PF example returns 1.2 kVA, matching the textbook single-phase example.

The same kVA, kW, and kVAr split is what you see on UPS nameplates as 'VA rating' versus 'W rating' and in utility tariffs as 'PF penalty threshold'.

When you also need the resistance of the wiring or the resistive part of the load, Ohm's Law Calculator returns V, I, R, and P for any two of the four so you can sanity-check the voltage drop across the run.

Five factors shape the apparent kVA on the result panel, plus two limitations worth knowing.

• • The formula assumes sinusoidal voltage and current at a single fundamental frequency. Distorted waveforms from large rectifier banks need a power-quality meter.
• • Real three-phase installations are rarely perfectly balanced, so the k * V * I / 1000 shortcut is a planning estimate.

In teaching, the single-frequency, balanced-load assumption matches textbook examples. In the field, treat the answer as a planning number and confirm with a power analyzer before specifying a UPS or generator.

According to OpenStax University Physics Volume 2, Section 15.4 - Power in an AC Circuit, average AC power equals rms current times rms voltage times cos(phi), so a 5% error in the RMS reading flows through to a 5% error in the real kW.

When the apparent kVA feeds a standby generator and you need a sizing recommendation in kVA or kW, Generator Size Calculator turns the same inputs into a recommended generator rating.