What is the electrical input power for a 0.5 HP device?

What is the electrical input power for a 0.5 HP device?

• A. 186 watts
• B. 746 watts
• C. 373 watts
• D. 1,492 watts

Answer: (D)

Power lets us connect what the device does (its useful mechanical output) with what it uses from electricity. A 0.5 horsepower output equals 0.5 × 746 watts, which is 373 W of mechanical power at the shaft. The electrical input power is related to this by the efficiency: P_in = P_out / η, where η is between 0 and 1. If you see 1,492 W listed as the input, that implies an efficiency of 373 W / 1,492 W ≈ 0.25, or 25%. That’s a very low efficiency, but it’s the only way to reconcile 0.5 HP of output with 1,492 W of electrical input. In many real motors you’d expect higher efficiency, which would give other input power values (for example about 373 W at 100% efficiency, or roughly 746 W at around 50% efficiency). The given option represents the input power under a highly inefficient assumption, which is why it matches the provided answer.

Power lets us connect what the device does (its useful mechanical output) with what it uses from electricity. A 0.5 horsepower output equals 0.5 × 746 watts, which is 373 W of mechanical power at the shaft. The electrical input power is related to this by the efficiency: P_in = P_out / η, where η is between 0 and 1.

If you see 1,492 W listed as the input, that implies an efficiency of 373 W / 1,492 W ≈ 0.25, or 25%. That’s a very low efficiency, but it’s the only way to reconcile 0.5 HP of output with 1,492 W of electrical input. In many real motors you’d expect higher efficiency, which would give other input power values (for example about 373 W at 100% efficiency, or roughly 746 W at around 50% efficiency). The given option represents the input power under a highly inefficient assumption, which is why it matches the provided answer.