One horsepower-hour equals approximately how many joules?

• A. 2.684 × 10^6 J
• B. 1.0 × 10^6 J
• C. 9.0 × 10^3 J
• D. 3.6 × 10^6 J

Answer: (A)

Think of energy as power times time. One horsepower-hour is the energy produced by one horsepower running for one hour. A mechanical horsepower is defined as about 745.7 watts. Since energy in joules equals power in watts times time in seconds, convert one hour to seconds (3600 s) and multiply: 745.7 × 3600 ≈ 2,684,520 J, which is about 2.684 × 10^6 J. This is why the correct value is 2.684 × 10^6 J. The other numbers don’t fit because they would require different power (or time) than one horsepower for one hour.