Horsepower ratings on electric motors indicate the motor's mechanical output power.

• A. Electrical input power
• B. Efficiency
• C. Thermal rating
• D. Mechanical output power

Answer: (D)

Horsepower ratings on electric motors reflect the shaft’s mechanical power delivered to a load. An electric motor converts electrical input into mechanical work, but some energy is always lost to heat and other inefficiencies, so the rating focuses on what the motor can actually deliver at the output shaft under rated conditions. Efficiency is the ratio of output power to input power, not the rating itself, and the thermal rating relates to temperature limits and insulation rather than how much power is produced. For example, a motor rated at 5 horsepower is designed to provide about 5 HP of shaft output; if its efficiency is 85%, the electrical input would be about 5 / 0.85 ≈ 5.88 HP (roughly 4390 watts).