Enter the horsepower, load (output power), and input power into the calculator to determine the motor efficiency of an electric motor.

Motor Efficiency Formula

The following formula is used to calculate the efficiency of an electric motor.

n = .7457 * HP * L / iP *100 
  • Where n is the efficiency (%)
  • HP is the motor horsepower (hp
  • L is the load (percentage of rated power i.e .50 for 50%)
  • iP is the input power (kW)

To calculate a motor efficiency, multiply the horsepower by the load ratio, divide by the input power, and finally multiply by 74.57.

Motor Efficiency Definition

What is motor efficiency? Motor efficiency is the ratio or percentage of input power that a motor, typically an electric one, can convert into useful output work or power.

Example Problem

How to calculate motor efficiency?

  1. First, determine the input power.

    For this example, we are analyzing an electric motor in a car that receives its power from a battery back. The input power is 85 kW.

  2. Next, determine the maximum horsepower.

    The maximum rated horsepower of the engine/motor is 150 hp.

  3. Next, determine the load.

    The percentage of the max horsepower that the engine is running at is .65 or 65%.

  4. Finally, calculate the motor efficiency.

    Using the formula above, the motor efficiency is found to be .7457 * 150 *.65 / 85 *100 = 85.53 %.

About Motor Efficiency

How efficient are electric motors? Electric motors typically run at 90% or more efficiency. These efficiencies are usually achieved at loads of around 75% load.

Why does motor efficiency increase with load? As the load increases the relative motion between the rotor and the stator magnetic fields increases, which leads to more efficient operating conditions.