HP to kVA Calculator
Find the apparent power a motor draws from its supply, or the largest motor a transformer or generator can carry, using efficiency and power factor instead of the 1 kVA = 1.34 HP shortcut.
A motor nameplate gives horsepower, which is mechanical output at the shaft. Transformers, generators and UPS units are rated in kVA, which is the apparent electrical power they can deliver. Converting between them takes two losses into account: the motor's efficiency and its power factor. Skip either and the answer comes out too small, which is how undersized supplies get specified.
HP to kVA Formula
η = motor efficiency as a decimal (91.7% = 0.917)
PF = power factor as a decimal
The formula works in three steps. Multiplying horsepower by 0.7457 gives the shaft output in kilowatts. Dividing by efficiency gives the real electrical power the motor takes in, because some of the input turns into heat in the windings and bearings. Dividing by power factor gives the apparent power, which is what the supply has to be sized for.
Why kVA Is Larger Than kW for a Motor
An induction motor needs current to build its magnetic field. That magnetizing current flows back and forth between the supply and the motor without doing useful work, but it still heats the transformer windings and the cables. Kilowatts (real power) count only the work-producing part. Kilovolt-amperes (apparent power) count all the current, which is why equipment that carries current is rated in kVA.
Power factor is the ratio between the two: kW ÷ kVA. Full-load power factor for a standard induction motor is commonly in the 0.8 to 0.9 range, and it falls well below that when the motor runs lightly loaded. The nameplate often lists it as "PF". When the plate does not show it, 0.85 is a reasonable planning figure for a fully loaded motor, but the nameplate value is always better.
The "1 kVA = 1.34 HP" Mistake
Many conversion tables and shared charts state that 1 kVA equals 1.341 HP. That number is simply 1,000 W divided by 745.7 W. It treats kVA as if it were kW and assumes a perfect motor with no losses and a power factor of exactly 1. No induction motor works like that.
With realistic figures the gap is large. At 90% efficiency and a 0.85 power factor, 1 kVA of supply runs only about 1.03 HP of motor output, not 1.34. Using the shortcut to size a transformer for a 75 HP motor would suggest about 56 kVA, when the motor actually draws about 69 kVA at full load (95.4% efficiency, 0.85 PF) before any allowance for starting.
Some calculators make the opposite simplification and drop efficiency while keeping power factor. That is closer, but it still understates the load by the motor's losses, typically 5 to 15%.
HP to kVA Table (Running Load)
The efficiencies below are the federal minimum nominal full-load values for enclosed four-pole (1,800 rpm) general-purpose motors in 10 CFR 431.25. Power factor is set at 0.85 for every size so the effect of efficiency is easy to see. Your motor's nameplate figures will shift these numbers slightly.
| Motor HP | Efficiency | Input kW | Running kVA | Amps at 480 V 3φ | Amps at 208 V 3φ |
|---|---|---|---|---|---|
| 1 | 85.5% | 0.87 | 1.03 | 1.2 | 2.8 |
| 2 | 86.5% | 1.72 | 2.03 | 2.4 | 5.6 |
| 3 | 89.5% | 2.50 | 2.94 | 3.5 | 8.2 |
| 5 | 89.5% | 4.17 | 4.90 | 5.9 | 13.6 |
| 7.5 | 91.7% | 6.10 | 7.18 | 8.6 | 19.9 |
| 10 | 91.7% | 8.13 | 9.57 | 11.5 | 26.6 |
| 20 | 93.0% | 16.04 | 18.87 | 22.7 | 52.4 |
| 25 | 93.6% | 19.92 | 23.43 | 28.2 | 65.0 |
| 50 | 94.5% | 39.46 | 46.42 | 55.8 | 128.8 |
| 100 | 95.4% | 78.17 | 91.96 | 110.6 | 255.3 |
These currents are calculated from the formula. They are not the values you use for wire and breaker sizing. For that, the National Electrical Code requires its own full-load current tables, which are deliberately higher. Our motor full-load amps chart explains the difference.
kVA to HP: What Motor Can a Supply Run?
The reverse question usually comes from someone with a fixed supply: a 50 kVA transformer, a generator, or a rotary phase converter. The formula gives the largest motor whose running load fits the rating. A 50 kVA supply with a 93% efficient motor at 0.85 power factor could, in theory, carry about 53 HP of running load.
In practice you should not plan to load a supply to its full rating with one large motor. An across-the-line start draws several times the running kVA for a few seconds, and the supply has to hold its voltage through that surge. Phase converter makers rate their units in horsepower for this reason: the limiting factor is the starting surge, not the running load. Treat the kVA to HP result as a ceiling for steady running, then check starting.
kVA to HP Chart (Running Load)
The reverse chart shows the largest motor whose running load fits a given kVA. It assumes a 0.85 power factor and 92% efficiency, typical of a mid-size premium-efficiency motor. Many online charts use 0.8 and 90%, which gives slightly lower figures, for example 4.8 HP for 5 kVA.
| Supply kVA | Motor HP (running) | Supply kVA | Motor HP (running) |
|---|---|---|---|
| 5 | 5.2 | 45 | 47.2 |
| 10 | 10.5 | 50 | 52.4 |
| 15 | 15.7 | 75 | 78.7 |
| 25 | 26.2 | 100 | 104.9 |
| 30 | 31.5 | — | — |
These are ceilings for steady running with one motor as the only load. Starting surge and any other connected load reduce the motor size a supply can realistically handle.
Running kVA vs Starting kVA
This calculator gives running kVA. Starting kVA is set by the code letter on the motor nameplate, which states locked-rotor kVA per horsepower. For example, a code G motor draws 5.6 to 6.29 kVA per HP at the instant of starting. A 10 HP code G motor therefore pulls roughly 56 to 63 kVA for a moment, against about 9.6 kVA running. Generators are especially sensitive to this. Our guide to what size generator a motor needs works through starting kVA step by step.
Standard Transformer Sizes
Transformers come in fixed ratings, so the calculated kVA is rounded up to the next available size. Common US ratings are listed below. The calculator suggests the next size up for running load only. It does not allow for other loads on the same transformer, future growth, or motor starting.
| Type | Common kVA ratings |
|---|---|
| Three-phase (dry-type and pad-mount) | 15 · 30 · 45 · 75 · 112.5 · 150 · 225 · 300 · 500 · 750 · 1000 |
| Single-phase (distribution and dry-type) | 10 · 15 · 25 · 37.5 · 50 · 75 · 100 · 167 |
Generator kW vs kVA Ratings
Three-phase generators are normally rated at a 0.8 power factor, so their kVA rating equals the kW rating divided by 0.8. A generator sold as 100 kW is a 125 kVA machine. When matching a motor to a generator, compare like with like: running kW against the generator's kW, running kVA against its kVA, and starting kVA against the generator's motor-starting capability from the manufacturer's data sheet.
Worked Examples
Single-phase 5 HP air compressor
Single-phase motors are generally less efficient than the three-phase motors in the table, so use the nameplate figure. A 5 HP motor at 85% efficiency and a 0.85 power factor on 230 V single-phase draws 5 × 0.7457 ÷ 0.85 ÷ 0.85 = 5.16 kVA, or 22.4 A running. A 10 kVA single-phase transformer covers that running load. Whether it holds voltage during the start, which for a code G motor is 28 to 31 kVA, depends on the transformer and the wiring, so check with the supplier or utility.
Three-phase 25 HP pump
At 93.6% efficiency and 0.85 power factor: 25 × 0.7457 ÷ 0.936 ÷ 0.85 = 23.4 kVA, about 28 A at 480 V. The next standard three-phase transformer is 30 kVA, which is only acceptable if the pump is the sole load and its starting surge is controlled, for example with a soft starter or VFD.
For the same motor in kilowatts, use the three-phase HP to kW calculator. For current alone, use HP to amps.
Frequently Asked Questions
For a real motor, about 1 kVA per HP while running. A 1 HP motor at 85.5% efficiency and 0.85 power factor draws 0.7457 ÷ 0.855 ÷ 0.85 = 1.03 kVA. The figure 0.746 kVA is only correct for a perfect motor at unity power factor.
About 1.0 to 1.1 HP of motor output with typical efficiency and power factor. The often-quoted 1.341 HP assumes no losses and a power factor of 1, which overstates what a supply can run by roughly 30%.
About 4.9 kVA running, assuming 89.5% efficiency and a 0.85 power factor. Starting kVA is much higher. A code G 5 HP motor briefly draws about 28 to 31 kVA.
No. kVA = HP × 0.7457 ÷ (efficiency × power factor) applies to both. The phase changes only the current: divide kVA × 1000 by the voltage for single-phase, or by 1.732 × voltage for three-phase.
Use the nameplate value. If it is missing, 0.85 is a reasonable planning figure for a fully loaded induction motor. Lightly loaded motors run at a lower power factor, so their kVA per useful horsepower is higher.
Not safely. Running kVA is the minimum. The transformer must also hold voltage while the motor starts, and it usually serves other loads. Use the running figure as a starting point, then add the motor's locked-rotor kVA and the rest of the connected load.
Because generators are rated at a 0.8 power factor. The kW rating is the real power the engine can deliver and the kVA rating, kW ÷ 0.8, is what the alternator windings can carry. A 100 kW generator is a 125 kVA generator.