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Calories are units of energy. Various definitions exist but fall into two broad categories. The first, the small calorie, or gram calorie (symbol: cal), is defined as the approximate amount of energy needed to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere.
A horsepower-hour (hph) is an outdated unit of energy, not used in the SI system of units.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):