Horsepower (hp)
Horsepower, symbol hp, is a non-SI unit of power introduced in the late 18th century by James Watt. He developed the term to market his steam engines by relating their work to that done by draft horses. One horsepower was standardized as 550 foot-pounds per second, or about 745.7 watts in mechanical terms. Despite not being metric, horsepower remains widely used in the automotive, aviation, and machinery industries, particularly in the United States and the United Kingdom. Its continued popularity is due to tradition and the practicality of relating to horsepower over abstract units, such as watts. Bridging from non-metric to metric units, the megawatt further illustrates the diversity and scope of power measurement in modern contexts.
Kilowatt (kW)
The kilowatt (kW) is 1,000 watts. As electricity became essential in the late 19th and 20th centuries, the kilowatt became the common unit for household appliances, motors, and power generation. Household energy use is measured in kilowatt-hours (kWh), which track energy consumption over time. Kilowatts are also used for engines, renewable energy, and national grids. By scaling up from the watt, the kilowatt lets us express real-world power needs and production more easily. As we consider other systems for measuring power, it is useful to explore both non-metric and higher-magnitude units such as horsepower and the megawatt.