kVA Calculator

kVA and kW measure the same load twice. kVA is apparent power — volts multiplied by amps, everything the supply has to carry. kW is real power — the part that actually turns a shaft or makes heat. The ratio between them is the power factor.

The difference exists because current in a coil lags behind voltage. That lagging portion still flows through your cable, your breaker and your alternator, heating all three, but it does no work and your meter does not bill you for it. Which is why a generator is rated in kVA and sold on kW, and why the two numbers on the same nameplate differ by exactly the 0.8 the industry assumes.

Horsepower brings in a second, unrelated loss. A motor nameplate quotes shaft output. Getting that output requires more electrical input, because no motor is perfectly efficient. So converting HP to kVA needs both corrections: divide by efficiency to get electrical kW, then divide by power factor to get kVA.

Three-phase is the third complication. All three lines carry current and the phases are 120 degrees apart, so with voltage measured line-to-line the power works out to √3 — about 1.732 — times line volts times line amps.

1. Step 1 Apparent power, single-phase kVA = volts × amps ÷ 1000
2. Step 2 Apparent power, three-phase kVA = √3 × line_volts × line_amps ÷ 1000 Line volts means 208, 240 or 480 — the voltage between two phases.
3. Step 3 kVA to kW kW = kVA × power_factor
4. Step 4 kW to kVA kVA = kW ÷ power_factor
5. Step 5 Horsepower to kVA kVA = HP × 0.7457 ÷ efficiency ÷ power_factor Two divisions, two different losses. Efficiency is the motor; power factor is the supply.
6. Step 6 Reactive power kVAR = √(kVA² − kW²) The three form a right triangle. Correcting power factor shrinks the kVAR side and pulls kVA down towards kW.
7. Step 7 Generator sizing generator_kVA = load_kVA × 1.25, rounded up to a size you can buy Sized on kVA, not kW, because the alternator carries the reactive current too.

Example: a 10 HP motor at 88% efficiency and 0.85 power factor. Shaft output is 7.46 kW, electrical input is 8.47 kW, and apparent power is 9.97 kVA. On a 480 V three-phase supply that is 12.0 A per line. A generator to start and run it wants to be about 12.5 kVA before you allow for starting current, which for a direct-on-line motor can be six times higher again.

kVA and kW measure the same load twice. kVA is apparent power — volts multiplied by amps, everything the supply has to carry. kW is real power — the part that actually turns a shaft or makes heat. The ratio between them is the power factor.

The difference exists because current in a coil lags behind voltage. That lagging portion still flows through your cable, your MCB and your alternator, heating all three, but it does no work and your meter does not bill you for it. This is exactly why generators across Europe are advertised in kVA: a 5 kVA set sounds larger than the 4 kW it actually delivers.

Horsepower brings in a second, unrelated loss. A motor nameplate quotes shaft output, and getting that output requires more electrical input because no motor is perfectly efficient. So converting HP to kVA needs both corrections: divide by efficiency to get electrical kW, then divide by power factor to get kVA. European motors are usually rated in kW rather than HP, but imported machinery is not.

Three-phase is the third complication. A 400 V supply is 400 V between any two lines, all three carry current, and the phases sit 120 degrees apart — so the power works out to √3, about 1.732, times line volts times line amps.

1. Step 1 Apparent power, single-phase kVA = volts × amps ÷ 1000
2. Step 2 Apparent power, three-phase kVA = √3 × line_volts × line_amps ÷ 1000 Line volts means 400 V, not the 230 V measured from a phase to neutral.
3. Step 3 kVA to kW kW = kVA × power_factor
4. Step 4 kW to kVA kVA = kW ÷ power_factor
5. Step 5 Horsepower to kVA kVA = HP × 0.7457 ÷ efficiency ÷ power_factor Two divisions, two different losses. Efficiency is the motor; power factor is the supply.
6. Step 6 Reactive power kVAR = √(kVA² − kW²) The three form a right triangle. Power-factor correction capacitors shrink the kVAR side and pull kVA down towards kW.
7. Step 7 Generator sizing generator_kVA = load_kVA × 1.25, rounded up to a size you can buy Sized on kVA, not kW, because the alternator carries the reactive current too.

Example: a 7.5 kW motor at 90% efficiency and 0.85 power factor draws 8.33 kW electrically and 9.8 kVA of apparent power. On a 400 V three-phase supply that is 14.1 A per line. A generator to start and run it wants to be about 12.5 kVA before allowing for starting current, which for a direct-on-line motor can be six times higher again.

• The load is assumed balanced across all three phases when three-phase is selected. An unbalanced load has to be worked out phase by phase, and the worst phase decides the cable.
• Voltage entered for three-phase is line-to-line. Entering the line-to-neutral figure understates the load by a factor of 1.73.
• Power factor is treated as a single constant. In reality it moves with load — an induction motor at a quarter load can fall to 0.5, which is a good reason not to oversize motors.
• Motor efficiency applies only to the horsepower conversion. It is a nameplate figure at full load, and drops off sharply below about 40% load.
• Starting current is not included. A direct-on-line induction motor draws six to eight times its running current for a few seconds, which is usually what sizes the generator rather than the running kVA.
• Generator sizing here applies a flat 25% margin on apparent power. A real specification would also account for step-load acceptance, harmonic content from variable-speed drives, and altitude and temperature derating.
• Horsepower means mechanical horsepower at 745.7 W. Some equipment quotes electrical or metric horsepower, both slightly different.

• The load is assumed balanced across all three phases when three-phase is selected. An unbalanced load has to be worked out phase by phase, and the worst phase decides the cable.
• Voltage entered for three-phase is line-to-line, so 400 V rather than the 230 V measured to neutral. Entering 230 V understates the load by a factor of 1.73.
• Nominal voltage is treated as exact. The harmonised supply tolerance is 230 V +10%/−6%, so real current can sit several percent either side of the calculation.
• Power factor is treated as a single constant. In reality it moves with load — an induction motor at a quarter load can fall to 0.5, which is a good reason not to oversize motors.
• Motor efficiency applies only to the horsepower conversion. IE-class efficiencies are quoted at full load and fall away sharply below about 40% load.
• Starting current is not included. A direct-on-line induction motor draws six to eight times its running current for a few seconds, which is usually what sizes the generator rather than the running kVA. Star-delta and soft starters cut it substantially.
• Generator sizing applies a flat 25% margin on apparent power. A real specification would also account for step-load acceptance, harmonic content from variable-speed drives, and altitude and temperature derating.