Beginner
Motor/Equipment Efficiency
Calculate electrical efficiency — input power, output power, and losses for motors, transformers, and other equipment.
Formulas
η = (P_out / P_in) × 100%
Efficiency as a percentage
P_in = P_out / η
Input power from output and efficiency
P_loss = P_in − P_out
Power losses
P_in(motor) = HP × 746 / η
Motor electrical input from HP
Variable Definitions
η=Efficiency(decimal or percent (%))
P_out=Output (mechanical) power(watts (W))
P_in=Input (electrical) power(watts (W))
P_loss=Power losses (heat)(watts (W))
HP=Horsepower(HP (1 HP = 746W))
Worked Example
50 HP motor at 93% efficiency. Find input power and losses.
- P_out = 50 × 746 = 37,300W
- P_in = 37,300 / 0.93 = 40,108W (40.1 kW)
- P_loss = 40,108 − 37,300 = 2,808W (2.8 kW of heat)
Answer: 40.1 kW input, 2.8 kW losses
When to Use
- •Calculating actual electrical input power from motor HP rating
- •Determining heat dissipation for electrical room cooling
- •Comparing motor efficiency classes (standard vs premium)
- •Energy cost calculations for motor-driven equipment
Standards & References
- NEMA MG-1 — Motor Efficiency Standards
- NEC 430 — uses FLC tables that assume standard efficiency