Horsepower to Kilowatts Calculator
Two numbers live on an old US-made compressor's nameplate, and they are not the same thing: the horsepower it delivers at the shaft, and the electrical power it pulls from the panel to do it. Convert the horsepower straight to kilowatts and the result is the first number, shaft output: useful, but not what a breaker, a generator or an electrician's load calculation actually needs. Getting from that nameplate HP to the real electrical-input kilowatts takes one more step: dividing by the motor's efficiency.
That second step matters most on exactly the kind of equipment prompting the question (an older compressor, a well pump, a shop air handler), where the only figure stamped on the tag is horsepower and a separate efficiency percentage from a motor datasheet or a replacement-part label. Skip the division and a sizing calculation ends up short by the efficiency loss, which for a well-worn or lower-efficiency motor can run into several percent of the total load.
kW = 25 × 0.7457 = 18.64 kW
This calculator performs the first step only: HP × 0.7457, shaft-output kilowatts. It does not know the motor's efficiency, so the electrical-input adjustment below has to be worked separately.
Shaft output and electrical input aren't interchangeable
Horsepower is a unit of mechanical power: specifically, the power available at the motor's output shaft to turn a compressor screw, a pump impeller or a fan wheel. Converting it to kilowatts with the standard 0.7457 factor only changes the unit; it doesn't change what's being measured. The result is still shaft-output power, not the electrical power the motor consumes to produce it.
No motor converts electrical input into shaft output at 100%. Some of that input is lost to winding resistance, friction in the bearings and magnetic losses in the iron core, and all of it leaves as heat rather than useful work. The nameplate efficiency figure, typically 80-96% depending on motor size and design, is what accounts for that gap, and it's the number that turns a shaft-power figure into an electrical-power figure.
Efficiency is often confused with power factor, but they describe different things. Efficiency compares real power out to real power in, and is what widens the gap between shaft-output kW and electrical-input kW covered here. Power factor instead compares real power to apparent power on an AC circuit, and it's what makes the current a motor draws higher than a plain P = VI calculation would predict. Both push a motor's actual electrical demand above the number a bare horsepower conversion suggests, but a full sizing calculation has to account for each separately rather than folding one into the other.
The two-step conversion
Step one is the familiar conversion: shaft-output kilowatts equal horsepower times 0.7457. Step two, which the calculator above does not perform, is dividing that shaft-output figure by the motor's efficiency, expressed as a decimal, to reach electrical-input kilowatts.
Skipping the second step and treating the shaft-power figure as the electrical draw understates the load on the branch circuit, the panel and any generator sized to carry it. The input figure is always the larger of the two.
The same two-step logic applies to any HP-rated equipment, not just compressors: well pumps, shop fans, conveyor drives and older single-phase motors all carry a horsepower rating that describes shaft output only. Whenever a datasheet or a replacement-motor label lists both HP and an efficiency percentage, the electrical-input kilowatts are one division away; when only HP is given and no efficiency figure exists anywhere on the equipment, a rough placeholder in the 85-90% band keeps the estimate close for most general-purpose industrial motors, though the real nameplate figure should replace it before any final sizing decision.
Worked example: 25 HP compressor motor
25 HP nameplate rating, 90% motor efficiency.
- 1
Convert HP to shaft-output kW
25 × 0.7457 = 18.6425 kW
- 2
Divide by 90% efficiency for electrical-input kW
18.6425 / 0.90 = 20.7139 kW
The compressor's shaft only ever sees 18.64 kW of mechanical work; the panel behind it has to supply 20.71 kW to make that happen. That 2.07 kW difference is the efficiency loss, not a rounding error.
Common NEMA motor sizes, shaft-output kW
Shaft-output power only, HP × 0.7457, with no efficiency adjustment applied. Treat these as reference estimates for common frame sizes, not a substitute for the nameplate figure on a specific motor, and remember the electrical-input draw for each row will be higher once its own efficiency is divided in.
| Horsepower | Shaft output kW |
|---|---|
| 1 HP | 0.75 kW |
| 2 HP | 1.49 kW |
| 5 HP | 3.73 kW |
| 10 HP | 7.46 kW |
| 15 HP | 11.19 kW |
| 20 HP | 14.91 kW |
| 25 HP | 18.64 kW |
| 30 HP | 22.37 kW |
| 50 HP | 37.29 kW |
| 100 HP | 74.57 kW |
Reference estimates only, not measured values. A real motor's electrical input depends on its own nameplate efficiency and power factor.
Further reading on motor power
Questions
Horsepower to kilowatts FAQ
Questions that come up once a nameplate HP figure has to become a real electrical-input number.
Why does a motor's electrical draw work out higher than its horsepower converted straight to kilowatts?
Because horsepower measures power delivered at the shaft, while the electrical input has to supply that shaft power plus everything lost to heat, friction and magnetic losses inside the motor. A 25 HP motor running at 90% efficiency needs 20.71 kW pulled from the panel to deliver 18.64 kW of shaft output. That 10% gap is exactly what the efficiency rating quantifies.
What is a typical efficiency range for motors by size?
As general reference, small fractional- and low-horsepower motors often run 75-85% efficient, mid-size industrial motors around 25 HP commonly land in the high 80s to low 90s, and large motors above 100 HP can reach 94-96%. These are typical NEMA-table ranges, not a substitute for the nameplate or datasheet efficiency figure of the actual motor being sized.
Does the horsepower on a motor's nameplate already include efficiency losses?
No. Nameplate horsepower is the rated shaft output the motor is designed to deliver, not the electrical power it consumes to produce that output. The efficiency figure, usually printed separately on the same nameplate, is what links the two: divide the shaft-power-equivalent kilowatts by that efficiency to find the actual electrical input.
How is service factor different from efficiency?
Service factor is a margin for how far above rated horsepower a motor can briefly be loaded without immediate damage, while efficiency describes how much of the electrical input at rated load actually becomes shaft power. A 25 HP motor with a 1.15 service factor can handle roughly 28.75 HP of shaft load intermittently. That margin has no direct bearing on the 90% efficiency figure used to size its electrical input.
When should I use input kW instead of output kW for sizing upstream equipment?
Anything upstream of the motor shaft (the branch circuit, the panel, a generator, a transformer) has to supply electrical input power, so input kW is the figure that belongs in that sizing math, not the shaft-output number a plain horsepower conversion produces. Output kW stays useful for comparing what the driven equipment, like a compressor pump end, actually receives, but it understates the demand a generator or breaker will see by exactly the efficiency loss.
Can two motors rated at the same horsepower draw different electrical input power?
Yes, horsepower only describes shaft output, so two motors rated at the same HP but different efficiencies pull different electrical input power for identical work. A 25 HP motor at 90% efficiency draws 20.71 kW, while a less efficient 25 HP motor at 85% efficiency draws roughly 21.93 kW for the same shaft output, a gap that grows more noticeable the harder the motor runs.