Definition: A kilowatt (symbol: kw) is a unit of power in the International System of Units (SI). The base unit of the kilowatt is the watt, which was named after Scottish inventor James Watt. As is consistent with SI units, the prefix kilo- means that a kilowatt equals one thousand watts or one thousand joules per second.

History/origin: The base unit of the kilowatt is the watt, which was named after a Scottish inventor named James Watt. This was first proposed in 1882 by Sir Charles William Siemens, an engineer and entrepreneur, who suggested the use of Watt's name as a unit of power. He defined the unit within the system of units being used at the time, and his definition was adopted in 1908.

Current use: Kilowatts are used worldwide, typically to express the output power of engines and the power of electric motors, tools, machines, and heaters. Electricity used by a home is typically measured in kilowatt hours, or kWh, meaning 1000 watts being applied over a period of one hour. Megawatt or gigawatt hours may be used in larger buildings or for industrial applications.

Definition: The unit horsepower (symbol: hp) is a unit of measurement of power (the rate at which work is done). Mechanical horsepower, also known as imperial horsepower, is defined as approximately 745.7 watts (550 ft·lbf/s), while metric horsepower is approximately 735.5 watts (75 kgf·m/s). Boiler horsepower, albeit a less common measurement than either imperial or metric horsepower, is used for rating steam boilers, and is equivalent to 34.5 pounds of water evaporated per hour at 212 degrees Fahrenheit, or 9809.5 watts. In addition, when rating electric motors, one horsepower is equal to 746 watts.

History/origin: The term horsepower was adopted in the late 18th century by James Watt to compare the output of steam engines with the power of draft horses. Watt was not the first person to compare the output of horses to that of engines. As early as 1702, Thomas Savery referenced horses when describing the output of an engine. It is believed that Watt built on this idea and introduced the term horsepower, largely in an effort to market his steam engine. The term was later expanded to include other types of output power such as the imperial and metric horsepower measurements commonly used today.