How to Convert Horsepower to Watts
Converting horsepower to watts is an essential calculation for engineers, automotive professionals, and anyone working with both traditional and modern power measurement systems. Horsepower (hp) is a legacy unit of power that has been used for over two centuries to rate the output of engines, motors, and other mechanical systems. The watt (W) is the SI unit of power, representing one joule per second, and is the international standard for all power measurements. One mechanical horsepower equals approximately 745.7 watts. This conversion is crucial in the automotive industry where the global shift toward electric vehicles demands expressing engine power in watts and kilowatts alongside traditional horsepower. Engineers designing hybrid systems must seamlessly convert between these units when combining electric motors (rated in watts) with combustion engines (rated in horsepower). Industrial equipment, pumps, and compressors are frequently specified in horsepower in North America but need watt equivalents for international markets and electrical system planning. Whether you are an automotive enthusiast comparing vehicle specifications, an engineer sizing an electrical supply, or a student studying power systems, mastering the horsepower-to-watt conversion is a fundamental technical skill.
Conversion Formula
To convert mechanical horsepower to watts, multiply the power in horsepower by 745.7. This factor comes from the original definition of one mechanical horsepower as 33,000 foot-pounds per minute, which equals 550 foot-pounds per second. Converting this to SI units: 550 foot-pounds per second × 1.35582 joules per foot-pound = approximately 745.7 watts. The exact value is 745.69987 watts per mechanical horsepower.
Watts = Horsepower × 745.7
5 horsepower = 3728.5 watts
Step-by-Step Example
To convert 5 horsepower to watts:
1. Start with the value: 5 horsepower
2. Multiply by the conversion factor: 5 × 745.7
3. Calculate: 5 × 745.7 = 3728.5
4. Result: 5 horsepower = 3728.5 watts
Five horsepower is a common power rating for residential riding lawn mowers and small outboard motors. In watts, this is approximately 3.73 kilowatts.
Understanding Horsepower and Watts
What is a Horsepower?
Horsepower was created by James Watt in approximately 1782 as a way to quantify the output of his steam engines in terms that mine and mill operators could understand. By measuring the work rate of mill horses, Watt determined that a horse could lift 33,000 pounds by one foot in one minute, and he defined this as one horsepower. This unit gave customers a concrete way to compare his engines to the horses they would replace. Horsepower became the dominant unit for rating engines throughout the Industrial Revolution and the subsequent age of the automobile. Despite the official adoption of the watt as the SI unit of power, horsepower remains in common use in the automotive, marine, and industrial sectors, particularly in North America.
What is a Watt?
The watt was named in honor of James Watt, the same inventor who created the horsepower unit. This naming is one of science's great ironies: the standardized SI unit that would eventually compete with horsepower bears the name of the man who invented horsepower. The watt was adopted as the SI unit of power in 1882 and is defined as one joule per second. It applies universally to all forms of power transfer, including electrical, mechanical, thermal, and electromagnetic. The watt's integration into the SI system, with its clean decimal prefixes (milliwatt, kilowatt, megawatt, gigawatt), has made it the preferred power unit for scientific and engineering work worldwide.
Practical Applications
Horsepower-to-watt conversion is essential for electric vehicle development, where legacy horsepower benchmarks must be matched or exceeded by electric motor watt ratings. Industrial facilities converting from older horsepower-rated equipment to modern metric-rated machines use this conversion for procurement and planning. Electrical engineers convert motor horsepower to watts when sizing electrical panels, transformers, and cables for industrial installations. HVAC professionals convert compressor horsepower to watts for energy efficiency calculations and electrical load planning. The renewable energy sector uses this conversion when comparing traditional generator output to wind turbine or solar system capacity.
Tips and Common Mistakes
The most critical mistake is not specifying which type of horsepower is being converted. Mechanical (imperial) horsepower uses 745.7 W, metric horsepower (PS) uses 735.499 W, and electrical horsepower uses exactly 746 W. Using the wrong conversion factor can introduce errors of 1-2%, which matters in engineering applications. Another common error is confusing peak horsepower with continuous (rated) horsepower. Many consumer products advertise peak horsepower, which can be significantly higher than sustained output. When converting for electrical system sizing, always use the continuous rating and add an appropriate safety margin. Remember that efficiency losses mean a motor rated at a certain horsepower actually draws more watts from the power supply than the conversion suggests.
Frequently Asked Questions
One mechanical (imperial) horsepower equals approximately 745.7 watts. One metric horsepower (PS) equals approximately 735.5 watts. One electrical horsepower equals exactly 746 watts. The mechanical horsepower is the most commonly used type in the United States, while metric horsepower is standard in continental Europe.