Horsepower-hours to Calories (hp⋅h to cal)

1 horsepower-hour equals 641,615.5683 calories (rounded).

1 horsepower-hour = 641,615.5683 calories

How to convert horsepower-hours to calories

To convert horsepower-hours to calories, multiply the number of horsepower-hours by 641,615.5683. The factor is rounded to 10 significant digits. To go the other way, multiply calories by 0.000001558565673 (rounded).

calories = horsepower-hours × 641,615.5683

Example: 10 horsepower-hours = 6,416,155.683 calories.

The 'calorie' on food labels is really a kilocalorie. The thermochemical calorie (4.184 J) is used here unless marked IT.

About these units

Horsepower-hour (hp⋅h)

US & ImperialTraditional unit (still used in some regions)

About 2.685 MJ (0.7457 kWh), the energy of one mechanical horsepower for an hour.

Multiplying a power by a time gives energy, and this unit does it with James Watt's horse. In the late 18th century Watt compared steam engines with the draft horses they replaced. After an experiment in 1782 that suggested a brewery horse delivered 32,400 foot-pounds per minute, he and Matthew Boulton rounded the figure to 33,000 foot-pounds per minute the next year. One mechanical horsepower is therefore about 745.7 W, and sustaining it for an hour gives roughly 2.685 MJ, or 0.7457 kWh.

The unit is now outdated and outside the SI. Railways still use it to settle accounts when locomotives are lent between companies. In one example, a 2,500 hp unit used for 12 hours is owed back as 30,000 hp-hours, repayable with a 3,000 hp unit for 10. A German variant, the Pferdestaerkenstunde, uses the metric horsepower of about 735.5 W and equals 2,647.7955 kJ, so one hp-hour is about 1.014 PSh.

1 horsepower-hour = 2,684,519.538 joules (approximately).

Sources: Wikipedia: Horsepower-hour; Wikipedia: Horsepower; Wikipedia: Conversion of units of energy

Calorie (cal)

ScientificTraditional unit (still used in some regions)

The thermochemical calorie: exactly 4.184 joules, the energy to raise one gram of water by one degree Celsius. Not the same as the food Calorie, which is a kilocalorie.

Latin calor, meaning heat, supplied the name. Nicolas Clement used it in lectures on heat engines between 1819 and 1824, and by 1852 Pierre Favre and Jean Silbermann applied it to the small unit based on a single gram of water warmed by one degree Celsius. Because the specific heat of water varies slightly with temperature, early calories differed from one laboratory to the next. A mean value near 4.1855 J was published for the 15 degree calorie in 1950, with an uncertainty of 0.0005 J.

Chemists settled the matter by definition. The thermochemical calorie is exactly 4.184 J, which is the figure used here, and NIST lists it with that exact value. The calorie is not an SI unit and is regarded as obsolete, since the ninth General Conference on Weights and Measures in 1948 preferred the joule for heat. One thousand of these calories make a kilocalorie, the larger unit used for food, and a slightly larger calorie, the International Table version of 4.1868 J, is used in steam tables.

1 calorie = 4.184 joules (exactly).

Sources: Wikipedia: Calorie; NIST SP 811, Appendix B.8 (energy conversion factors)

Related conversions

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