How to Calculate Motor Power: kW, HP and Torque Explained
Selecting the wrong motor size wastes energy or burns out equipment. This guide explains the motor power formula, how to convert between kW and HP, account for efficiency losses, and size a motor correctly for your application.
Electric motor power calculation is one of the most common tasks for plant engineers and maintenance teams. Whether you are sizing a new pump motor, checking if an existing motor is overloaded, or preparing a power budget for a facility, you need to calculate the required shaft power — and then account for motor efficiency to get the electrical input power.
This guide covers the full calculation: from torque and speed to shaft power, from shaft power to electrical input power, and how to convert between kilowatts (kW) and horsepower (HP).
The Core Motor Power Formula
Motor shaft power is calculated from torque and rotational speed:
**P (kW) = T × N / 9,550**
Where: - P = shaft power in kilowatts (kW) - T = torque in Newton-metres (N·m) - N = rotational speed in revolutions per minute (RPM) - 9,550 = unit conversion constant (60,000 / 2π × 1,000)
In horsepower: **P (HP) = T × N / 5,252**
Where T is in lb·ft and N is in RPM.
- T in N·m, N in RPM → P in kW: use divisor 9,550
- T in lb·ft, N in RPM → P in HP: use divisor 5,252
- 1 HP = 0.746 kW | 1 kW = 1.341 HP
Worked Example 1: Conveyor Drive Motor
A conveyor drive requires 320 N·m of torque at 1,450 RPM (a standard 4-pole motor speed at 50 Hz).
**Shaft Power = 320 × 1,450 / 9,550 = 48.6 kW**
The motor nameplate will typically show 55 kW (next standard frame size above 48.6 kW). You size up to the next standard rating — never down.
| Standard Motor Rating (kW) | Typical Full-Load Torque at 1,450 RPM (N·m) | HP Equivalent |
|---|---|---|
| 7.5 kW | 49 N·m | 10 HP |
| 11 kW | 72 N·m | 15 HP |
| 15 kW | 99 N·m | 20 HP |
| 22 kW | 145 N·m | 30 HP |
| 30 kW | 198 N·m | 40 HP |
| 37 kW | 244 N·m | 50 HP |
| 45 kW | 296 N·m | 60 HP |
| 55 kW | 362 N·m | 75 HP |
| 75 kW | 494 N·m | 100 HP |
| 90 kW | 593 N·m | 120 HP |
Accounting for Motor Efficiency
The shaft power is the mechanical output. The electrical input power is always higher because motors are not 100% efficient. Typical IE2 and IE3 motor efficiencies range from 88% to 96%.
**Electrical Input Power (kW) = Shaft Power (kW) / Motor Efficiency**
For the conveyor example above with a 93% efficient motor: **Input Power = 48.6 / 0.93 = 52.3 kW**
This is the power drawn from the electrical supply. Use this figure for cable sizing, switchgear rating, and energy cost calculations.
| Motor Rating | IE2 Efficiency (typical) | IE3 Efficiency (typical) |
|---|---|---|
| 7.5 kW | 89.5% | 91.0% |
| 15 kW | 91.8% | 93.0% |
| 22 kW | 92.7% | 93.6% |
| 37 kW | 93.3% | 94.2% |
| 55 kW | 93.9% | 94.7% |
| 75 kW | 94.3% | 95.0% |
| 90 kW | 94.6% | 95.2% |
Worked Example 2: Pump Motor Sizing
A centrifugal pump delivers 50 m³/h of water against a total head of 35 m. Pump hydraulic efficiency is 72%.
**Hydraulic Power = ρ × g × Q × H / 1,000** = 1,000 × 9.81 × (50/3,600) × 35 / 1,000 = **4.77 kW**
**Shaft Power Required = Hydraulic Power / Pump Efficiency** = 4.77 / 0.72 = **6.62 kW**
**Motor Input Power (93% efficient motor)** = 6.62 / 0.93 = **7.12 kW**
Select a **7.5 kW motor** (next standard size). Add a 10–15% service factor for fouling and start-up loads.
How to Calculate Starting Current and Torque
Standard squirrel cage induction motors draw 5–7× full-load current at direct-on-line (DOL) start. This matters for fuse sizing, cable selection, and whether a star-delta or VFD starter is needed.
**Full Load Current (A) = P (kW) × 1,000 / (√3 × V × PF × η)**
For a 55 kW, 415V, 3-phase motor with PF = 0.86, η = 0.94: FLC = 55,000 / (1.732 × 415 × 0.86 × 0.94) = **99 A**
Starting current (DOL) ≈ 6 × 99 = **594 A** — size MCB and cables accordingly, or use a soft starter / VFD to limit inrush.
- DOL starting: simplest, highest inrush current (5–7× FLC)
- Star-delta: reduces starting current to ~33% of DOL, but torque also drops to 33%
- Soft starter: smooth ramp-up, current limited to 2–4× FLC
- VFD: best control, lowest inrush, allows variable speed operation
Common Motor Power Calculation Mistakes
These errors consistently lead to under-sized or over-sized motors in industrial installations:
- Using average load instead of peak load — motors must handle peak torque, not average torque
- Ignoring transmission losses — belt drives lose 2–5%, gearboxes lose 2–8% per stage
- Forgetting service factor — add 10–15% buffer for real-world variations in load and friction
- Selecting motor for 60 Hz when plant runs on 50 Hz — speed and torque will be different
- Not checking ambient temperature — motors derate above 40°C (typically 1% per °C above 40°C)
- Ignoring altitude — motors above 1,000 m need derating due to lower air density for cooling
Use the Free Motor Power Calculator
Our free Motor Power Calculator handles both directions of the calculation: enter torque and RPM to get shaft power, or enter power and RPM to get the required torque. It also converts between kW and HP automatically.
Use it alongside the Cable Size Calculator to complete your motor installation design — once you have the motor kW and voltage, you can size the supply cable and determine the correct fuse and circuit breaker rating.