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

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

• A. 994.6 watts
• B. 746 watts
• C. 149 watts
• D. 932.5 watts

Answer: (A)

The key idea is that input power relates to output power through efficiency: Output power = Efficiency × Input power, so Input power = Output power / Efficiency. A horsepower is 746 watts, so the motor’s output at 1 hp is 746 W. With 75% efficiency, the required input power is 746 W ÷ 0.75 = approximately 994.6 W. The 12-volt rating isn’t needed for this calculation unless you want current, since power and efficiency alone determine the input. Therefore, about 994.6 watts of input power are required.

The key idea is that input power relates to output power through efficiency: Output power = Efficiency × Input power, so Input power = Output power / Efficiency. A horsepower is 746 watts, so the motor’s output at 1 hp is 746 W. With 75% efficiency, the required input power is 746 W ÷ 0.75 = approximately 994.6 W. The 12-volt rating isn’t needed for this calculation unless you want current, since power and efficiency alone determine the input. Therefore, about 994.6 watts of input power are required.