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

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## 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?

**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.

**Next, determine the maximum horsepower.**The maximum rated horsepower of the engine/motor is 150 hp.

**Next, determine the load.**The percentage of the max horsepower that the engine is running at is .65 or 65%.

**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.