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The erg is a unit of energy and work equal to 10−7 joules. It originated in the centimetre–gram–second (CGS) system of units. It has the symbol erg. The erg is not an SI unit. Its name is derived from ergon (’έργον) a Greek word meaning work or task.
A horsepower-hour (hph) is an outdated unit of energy, not used in the SI system of units. The unit represents an amount of work a horse is supposed capable of delivering during an hour (1 (one) horsepower integrated over a time interval of an hour). Based on differences in the definition of what constitutes the 'power of a horse', a horsepower-hour differs slightly from the German "Pferdestärkenstunde" (PSh):