Convert Horsepower-Hours to Millijoules
Convert Horsepower-Hours (hp·h) to Millijoules (mJ) instantly and accurately.
Conversion Formula
mJ = hp·h × 2684519538
About Horsepower-Hours
A horsepower-hour (hp·h) is the energy from one mechanical horsepower (745.6999 W) for one hour - exactly 2,684,519.54 J ≈ 2.685 MJ. James Watt coined the horsepower to compare his steam engines with mine horses; billing in hp·h preceded the kilowatt-hour tariff. The metric horsepower (PS = 75 kgf·m/s = 735.499 W) gives a slightly smaller metric-hp·h = 2,647,795 J, used in European and Japanese engine datasheets. Today the hp·h appears in marine engineering logs, mining energy accounting, and vintage industrial equipment. 1 hp·h = 2.6845 MJ = 0.74570 kWh = 641.19 kcal.
About Millijoules
A millijoule (mJ) is 10⁻³ J - the scale of surgical lasers, spark ignition, and the kinetic energy of small everyday objects. Nd:YAG lasers for ophthalmology and tattoo removal deliver 10-200 mJ per pulse. The minimum spark energy to ignite methane-air is ~0.29 mJ; an automotive spark plug discharges 30-60 mJ per cycle. A 2 g mosquito flying at 1 m/s has kinetic energy ½ × 0.002 × 1² = 1 mJ. Electrocautery monopolar pulses deliver 50-500 mJ to coagulate blood vessels. 1 mJ = 10⁻³ J = 0.2390 cal.
Quick Reference Table
| Horsepower-Hours (hp·h) | Millijoules (mJ) |
|---|---|
| 1 hp·h | 2.685 × 109 mJ |
| 2 hp·h | 5.369 × 109 mJ |
| 5 hp·h | 1.342 × 1010 mJ |
| 10 hp·h | 2.685 × 1010 mJ |
| 25 hp·h | 6.711 × 1010 mJ |
| 50 hp·h | 1.342 × 1011 mJ |
| 100 hp·h | 2.685 × 1011 mJ |