About Power Conversion
Power measures the rate at which energy is transferred or work is done, distinguishing it from energy itself, which measures a total amount. A light bulb rated at 60 watts uses energy at a constant rate of 60 joules per second, whether it runs for a minute or a day; the wattage doesn't change, but the total energy consumed does. This distinction matters when converting between power units used in different industries, since mixing up power and energy conversions is one of the most common unit-conversion errors.
The watt, and its larger multiple the kilowatt, form the metric standard for electrical and mechanical power. Horsepower remains the traditional unit for engines and motors, especially in the automotive industry, with slightly different definitions used historically in mechanical versus electrical (or metric) horsepower. This converter covers watts, kilowatts, and horsepower using precise standard definitions, useful for comparing an engine's horsepower rating to an electric motor's wattage, or converting appliance power ratings between regions.
Power unit conversion comes up constantly when comparing specifications across industries that default to different units: an electric vehicle's motor rated in kilowatts needs converting to horsepower to compare against a traditional gasoline engine, and international appliance wattage ratings need checking against local electrical standards. Because power ties directly into both mechanical and electrical engineering, understanding how watts, kilowatts, and horsepower relate is useful well beyond a single industry, from automotive shopping to home appliance specifications to industrial equipment ratings.
When comparing power specifications across systems, it helps to remember that horsepower and kilowatts are reasonably close in magnitude, roughly 1.34 horsepower per kilowatt, so a rough mental conversion (multiply kilowatts by about 1.3, or divide horsepower by about 0.75) can serve as a useful sanity check before relying on the precise converted figure for a purchasing or engineering decision.
Common Power Measurements
The watt (W) is the SI derived unit of power, defined as one joule of energy transferred per second.
| Unit | Symbol | Equal to |
|---|
| Watt | W | 1 W (SI derived unit) |
| Kilowatt | kW | 1,000 W |
| Megawatt | MW | 1,000,000 W |
| Mechanical horsepower | hp | 745.7 W |
| Metric horsepower | PS | 735.5 W |
Conversion Formulas
- Watts to horsepower: divide by 745.7
- Horsepower to watts: multiply by 745.7
- Kilowatts to horsepower: multiply by 1.34102
- Horsepower to kilowatts: multiply by 0.7457
- Watts to kilowatts: divide by 1,000
Practical Examples
- A 100-horsepower engine produces about 74,570 W, or 74.57 kW.
- A 1,500 W microwave is about 2.01 horsepower, since 1,500 ÷ 745.7 ≈ 2.01.
- A 5.0 kW solar inverter equals about 6.71 horsepower, since 5.0 × 1.34102 ≈ 6.71.
- A 250-horsepower car engine produces about 186.4 kW, since 250 × 0.7457 ≈ 186.4.
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Frequently asked questions
What is the difference between power and energy?
Power is the rate at which energy is used or transferred, measured in watts, while energy is the total amount transferred over time, measured in joules or kilowatt-hours. A high-power device uses energy quickly; running it longer increases total energy used, not its power rating.
How many watts are in one horsepower?
One mechanical horsepower equals approximately 745.7 watts. A slightly different unit, metric horsepower (PS), equals about 735.5 watts; the two are close but not identical.
Why do car engines use horsepower instead of watts?
Horsepower has been the traditional unit for engine output since James Watt popularized it in the 18th century to compare steam engines against draft horses, and the automotive industry, especially in the US and UK, has kept the convention ever since, even though watts and kilowatts are used in most other power contexts.
What is the difference between mechanical and metric horsepower?
Mechanical (imperial) horsepower equals 745.7 watts and is used in the US and UK, while metric horsepower (PS or CV) equals about 735.5 watts and is used in much of continental Europe and Asia for vehicle specifications. The difference is small, about 1.4%, but can cause slight mismatches when comparing published specs.
How is power converted to energy?
Multiply power by the time over which it's applied. A 1,000-watt (1 kW) appliance running for 1 hour consumes 1 kilowatt-hour of energy, which is why kilowatt-hours, not watts, appear on electricity bills.
History
The watt is named after James Watt, whose late 18th-century steam engine improvements made him want a way to market his engines' output against the working animals they replaced, leading him to define horsepower as a marketing benchmark. Ironically, the watt (the SI unit) was formalized later, in the 1880s, as electrical engineering matured and needed a scientifically rigorous power unit tied to the volt and ampere rather than a horse's exertion. Horsepower itself was never perfectly standardized, splitting into slightly different mechanical, metric, and boiler horsepower definitions depending on the country and industry. This is why automotive and industrial specifications today still routinely require converting between horsepower and kilowatts to compare engines built under different national standards.
Why Convert Power Units?
Engine, motor, and appliance power ratings are split between horsepower and watts depending on the country and industry, so converting between them is essential for comparing specs internationally. Since power relates directly to energy consumption, see our Energy converters and our Electricity converters.