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Online horsepower calculator can be used to estimate the power output. It takes into account factors such as force, distance, and time.

Horsepower is a unit of measurement of power developed by engineer James Watt in the late 18^{th} century. Although its original purpose was to compare the output of steam engines with the power of horses (hence its name), it has since been adopted as a unit of measurement for all sorts of engines used to power things such as vehicles, lawn mowers, boats, chainsaws, and airplanes.

Horsepower is commonly used today to refer to the potential work output of a vehicle's engine, and as such, is a widely cited measurement for performance comparisons. Although horsepower is an important metric for doing so, it should not be the only consideration in determining a vehicle's ability as there are other factors such as power-to-weight ratio, torque, drivetrain, and forced induction.

When measuring horsepower output of anything, to ensure precise comparisons between them, it is important to be consistent in the methodology used to determine horsepower.

Horsepower is not recognized in the International System of Units (SI); SI uses a measurement of power (a rate at which energy is generated or used) called a watt, which is named after James Watt. A watt is defined as one joule per second and is generally used to quantify a rate of energy transfer for lower rates of power consumption such as lightbulb or smartphone charging. On the other hand, kilowatts are commonly used for larger scale measures such as appliances and devices like refrigerators or servers. Energy is often presented using units of kilowatt hours (kWh), usually within the context of electric utilities delivering energy to household consumers.

The British Thermal Unit (BTU) is another unit of energy, and one unit of BTU is the amount needed to heat one pound of water by one degree Fahrenheit. Coincidentally, one BTU is also roughly the amount of energy released by burning one match. BTU is commonly used to compare the energy inherent in different fuels. 1 BTU is equivalent to 0.293 watt-hour or 1,055 joules.

When the term horsepower is used, people are probably referring to mechanical horsepower. However, there are other lesser known definitions of the word.

**Mechanical Horsepower, hp(I)**—550 foot-pounds per second, approximately 745.7 watts. The most common definition of horsepower, mechanical horsepower, is the horsepower James Watt invented in 1782.**Metric Horsepower, hp(M)**—75 kgf-m per second, approximately 735.499 watts**Boiler Horsepower, hp(S)**—34.5 pounds of water evaporated per hour at 212 degrees Fahrenheit, approximately 9,809.5 watts. This definition is mainly used to denote a boiler's capacity to deliver steam to a steam engine.**Electrical Horsepower**—746 watts. This definition is mainly used for electrical machines.