What Is Power?
Power is the rate at which energy is transferred or converted. It measures how quickly work is done. The SI unit of power is the watt (W), defined as one joule per second. Power is a fundamental concept in physics, electrical engineering, mechanical engineering, and thermodynamics. Understanding power units is crucial when designing electrical systems, selecting motors, or comparing vehicle performance.
Why Power Conversion Matters
Different industries use different power units. Electrical systems use watts and kilowatts, automotive specs list horsepower, HVAC systems rate capacity in BTU/hr, and scientific contexts may use ergs per second. When comparing a car engine's horsepower to an electric motor's kilowatt rating, or converting HVAC BTU/hr to electrical watts for sizing circuits, accurate power conversion is essential.
Key Power Units
A watt equals one joule per second and is the SI standard. A kilowatt (kW) is 1,000 watts, commonly used for electrical appliances and electric vehicles. Mechanical horsepower (hp) equals approximately 745.7 watts and is traditional for engines. BTU per hour measures heating/cooling capacity. Ergs per second (a CGS unit) equals 10⁻⁷ watts. Calories per second relate power to heat transfer rates.
Best Practices
When comparing motors, note that there are different definitions of horsepower: mechanical (745.7 W), metric (735.5 W), and electrical (746 W). For HVAC sizing, 1 ton of cooling = 12,000 BTU/hr ≈ 3.517 kW. When sizing electrical circuits, convert all loads to watts before summing. This converter uses the mechanical horsepower definition by default.
