A 1 HP, 24 volt DC electric motor that is 80 percent efficient requires 932.5 watts. How much power will a 1 HP, 12 volt DC electric motor that is 75 percent efficient require?

A 1 HP, 24 volt DC electric motor that is 80 percent efficient requires 932.5 watts. How much power will a 1 HP, 12 volt DC electric motor that is 75 percent efficient require?

• A. 746 watts
• B. 932.5 watts
• C. 994.6 watts
• D. 1,200 watts

Answer: (C)

Power in relates to power out through efficiency: output power = efficiency × input power. So to deliver 1 horsepower, which is 746 watts, at 75% efficiency you need more input power. Compute input power as 746 ÷ 0.75 = 994.666... watts, about 994.6 watts. The 12-volt rating isn’t needed for the input-power calculation itself, but it would imply a current of roughly 994.6 ÷ 12 ≈ 83 A. The earlier motor example shows consistency: input power of 932.5 W at 80% efficiency yields 746 W output (0.8 × 932.5 = 746).

Power in relates to power out through efficiency: output power = efficiency × input power. So to deliver 1 horsepower, which is 746 watts, at 75% efficiency you need more input power. Compute input power as 746 ÷ 0.75 = 994.666... watts, about 994.6 watts. The 12-volt rating isn’t needed for the input-power calculation itself, but it would imply a current of roughly 994.6 ÷ 12 ≈ 83 A. The earlier motor example shows consistency: input power of 932.5 W at 80% efficiency yields 746 W output (0.8 × 932.5 = 746).