How to Convert Watts to Horsepower
Converting watts to horsepower bridges the modern SI power measurement system with the traditional unit that has been used to rate engines and motors for over two centuries. The watt (W) is the SI unit of power, defined as one joule per second, and is the international standard for measuring all forms of power. Horsepower (hp) is a legacy unit of power originally conceived by James Watt to compare steam engine output to the work capacity of draft horses. One watt equals approximately 0.00134102 horsepower (mechanical). This conversion is essential for automotive engineers expressing electric motor output, for industrial equipment manufacturers providing specifications in both units, and for consumers comparing the power of electric and gasoline-powered equipment. The automotive industry frequently uses both units: electric vehicles are rated in kilowatts while combustion engines are rated in horsepower, so comparison shopping requires conversion. Whether you are evaluating the output of an electric motor, comparing power tools, or understanding vehicle performance specifications, converting watts to horsepower connects the precision of the metric system with the intuitive familiarity of horsepower.
Conversion Formula
To convert watts to mechanical horsepower, multiply the power in watts by 0.00134102. This factor is the reciprocal of 745.69987, which is the exact number of watts in one mechanical horsepower. The mechanical horsepower was originally defined by James Watt as 33,000 foot-pounds of work per minute, which translates to approximately 745.7 watts. Note that there are different definitions of horsepower (mechanical, electrical, metric, and boiler), each with slightly different watt equivalents.
Horsepower = Watts × 0.00134102
5 watts = 0.0067051 horsepower
Step-by-Step Example
To convert 5 watts to horsepower:
1. Start with the value: 5 watts
2. Multiply by the conversion factor: 5 × 0.00134102
3. Calculate: 5 × 0.00134102 = 0.0067051
4. Result: 5 watts = 0.0067051 horsepower
Five watts is a very small amount of power in horsepower terms. A more practical example: 7,457 watts equals approximately 10 horsepower, the output of a small outboard boat motor.
Understanding Watts and Horsepower
What is a Watt?
The watt is named after James Watt (1736-1819), the Scottish inventor who revolutionized the steam engine and introduced the concept of horsepower. Ironically, the SI unit named in his honor has largely supplanted the horsepower unit he popularized. The watt was adopted as the SI unit of power in 1882 by the British Science Association and was incorporated into the International System of Units in 1960. Defined as one joule per second, the watt is applicable to all forms of power: electrical, mechanical, thermal, and radiant. From milliwatts in microelectronics to gigawatts in nuclear power plants, the watt provides a universal, decimal-friendly framework for measuring power across all scales.
What is a Horsepower?
Horsepower was conceived by James Watt in the late 18th century as a marketing tool to help potential buyers of his improved steam engines understand their capability compared to horses. Through experiments with mill horses, Watt estimated that a horse could do 33,000 foot-pounds of work per minute, and he defined one horsepower accordingly. This somewhat generous estimate ensured that his engines would always favorably compare to horses. Despite being based on an informal experiment and a deliberately conservative comparison, horsepower became the standard unit for rating engines worldwide. It remains in widespread use in the automotive, marine, and industrial sectors, particularly in the United States, over 200 years after its inception.
Practical Applications
Watts-to-horsepower conversion is critical in the automotive industry, where electric vehicles are rated in watts or kilowatts but must be compared to traditional horsepower ratings that consumers understand. Industrial motor specifications often include both units to serve global markets. Power tool manufacturers may list wattage in one market and horsepower in another. HVAC system specifications sometimes use both watts and horsepower for compressor and fan motor ratings. Marine and aviation industries use horsepower for engine ratings but watts for auxiliary electrical systems, requiring conversion for total power budgeting. Agricultural equipment manufacturers provide both units for tractors and machinery sold in different markets.
Tips and Common Mistakes
The biggest source of confusion is that there are multiple definitions of horsepower. Mechanical (or imperial) horsepower equals approximately 745.7 watts, while metric horsepower (PS, or Pferdestarke) equals approximately 735.499 watts, and electrical horsepower equals exactly 746 watts. Always verify which type of horsepower is being used. Another common mistake is applying this conversion to voltage or current instead of power. Horsepower is strictly a power measurement. When comparing electric and gasoline vehicles, remember that electric motors deliver maximum torque instantly while combustion engines reach peak power at specific RPMs, so raw horsepower numbers alone do not tell the complete performance story.
Frequently Asked Questions
One thousand watts (1 kW) equals approximately 1.341 mechanical horsepower. This is a useful benchmark: roughly speaking, one kilowatt is about one and a third horsepower. For metric horsepower (PS), 1,000 watts equals approximately 1.3596 PS.